If there are no past and future lives and everything ends at the moment of death. Why should we bother with anything at all?
Why would the Buddha teach for 45 years if all our problems are definitely solved at the moment of death?
Life does not end with the moment of death. A new birth is taken based on the last thought that came at the time of death. If you it is suicide, the thoughts towards the end of your life are not positive. Therefore your future existence will not be a good one. It will be one of misery and pain.
The Buddha taught the realism of how misery arises and the way out of it. By practicing Buddhism you can get out of the pain in this lifetime and the next and also perhaps totally eradicate it. Total eradication happens when you are extinguished, which does not happen through suicide, but through the releasing causality (Dependent Origination, 4 Noble Truths, Conditional Relations).
All your problem are not solved at death. Death does not solve any problems but creates more, in terms misery to those you leave behind. The real solution is to understand causality thereby understand how your problems originate and do the needful so that the problems do not arise. You will find the unpleasantness of most of your problems are psychological and can be addressed change of your perspective and outlook of the world. One such change of paradigm is understanding the 3 Universal Characteristic which will intern let you understand causality. Armed with the wisdom of causality your perspective and outlook of the world will change. This means you can keep your mind free from worry and problems.
Getting this understanding of causality is not dogmatic or esoteric. the objective of the Buddhist practice is to understand causality so you can overcome your problems, misery or general unsafateryness of life. Anyone who practices Buddhism walks the Noble 8 Fold Path, is the how you find the way out of misery. The method to get the understanding of causality, the method to come out of all your misery or walking the Noble 8 Fold Path is also organised as the 3 Fold Training.
There is no real difference between:
this current moment in life with future moments afterwards with Nirvana as the climax
this current life with a string of lives afterwards with Nirvana as the climax.
The practitioner of the Buddha's teaching, done without ego, needs no future incentive.
We "should" practice to become enlightened and help others achieve enlightenment in this very life.
If you are sick or feeling unwell, why see a doctor?
If you live in a cave, why live in a clean modern house?
If you can walk, why drive a car or fly in an airplane?
If you are hungry, why eat?
If you are thirsty, why drink?
If there are no past and future lives, why should we practice Buddhism?
For happiness, ease & well-being.
The Buddha is called the 'Spiritual Doctor' and taught:
"In the past & now, I teach about suffering & the cessation of suffering".(MN 22)
The Buddha taught his Dhamma & goal are visible & experience in the here-&-now.
Buddhism is practised for Nibbana, which is called the "highest happiness".
If there are no past and future lives, we practice Buddhism for true happiness.
It is ideal to practise Buddhism in this life rather than wait for a future life.
In the pali canon, the Kesaputtiya sutta have the Buddha explaining the benefits of practicing high virtue by those who may believe in the after life, who may not believe it, who may believe there's no karma law, and who may not believe there's karma law.
He states that a virtuous person who practices virtue and the brahma-viharas have four assurances in his life here and now:
“This noble disciple, Kālāmas, whose mind is in this way without enmity, without ill will, undefiled, and pure, has won four assurances in this very life.
“The first assurance he has won is this: ‘If there is another world, and if there is the fruit and result of good and bad deeds, it is possible that with the breakup of the body, after death, I will be reborn in a good destination, in a heavenly world.’
“The second assurance he has won is this: ‘If there is no other world, and there is no fruit and result of good and bad deeds, still right here, in this very life, I maintain myself in happiness, without enmity and ill will, free of trouble.
“The third assurance he has won is this: ‘Suppose evil comes to one who does evil. Then, when I have no evil intentions toward anyone, how can suffering afflict me, since I do no evil deed?’
“The fourth assurance he has won is this: ‘Suppose evil does not come to one who does evil. Then right here I see myself purified in both respects.’
-- AN 3.65
Similarly, in another sutta, the Buddha explains how a wise man considers the teachers that advocated nihilism. By understanding that after death there is no consequences for their actions, these teachers apparently felt themselves free to do whatever they wanted, including evil things.
“About this a wise man considers thus: ‘If there is no other world, then on the dissolution of the body this good person will have made himself safe enough. But if there is another world, then on the dissolution of the body, after death, he will reappear in a state of deprivation, in an unhappy destination, in perdition, even in hell. Now whether or not the word of those good recluses and brahmins is true, let me assume that there is no other world: still this good person is here and now censured by the wise as an immoral person, one of wrong view who holds the doctrine of nihilism. But on the other hand, if there is another world, then this good person has made an unlucky throw on both counts: since he is censured by the wise here and now, and since on the dissolution of the body, after death, he will reappear in a state of deprivation, in an unhappy destination, in perdition, even in hell. He has wrongly accepted and undertaken this incontrovertible teaching in such a way that it extends only to one side and excludes the wholesome alternative.’
-- MN 60
I am not sure where you get the idea that there are no past or future lives. Sakyamuni wasn't concerned about past or future lives generally although he even mentioned past lives. He wanted to focus on this very moment. Just this one right here. Let's see if we can master our insecurities and infirmities for one second and find a perfect moment. That doesnt mean the other moments dont exist or matter.
Why? Thats a good question and it alludes to the broader thought - what is our purpose? We enter this world with basic needs and a basic way to resolve them. We're uncomfortable so we cry. We are self-serving creatures from the start. "Me" thinking.
"Me" thinking is fine for an isolated creature in a state of nature at the top of the food chain able to deal with its' environment. But we are a frail species that flourishes through COOPERATIVE efforts. Operationally as a species we arent loners but social creatures - which requires more thoughts about one another lest the freedom to swing my fist interferes with my neighbors nose. As a species we thrive on "WE" thinking more than "ME".
I am a behaviorist by profession and I find the biological evidence intriguing in this discussion. Our own brains are structured the same as other social creatures who we revere for their collaberative lifestyles and close-knit family ties. The corvids (crows, ravens), the orca, dolphins, elephants, primates - all species that have complicated frontal cortex centers for social communication and show remarkably human-like tendencies in empathy such as mourning their dead and placing their own safety at risk to help their family. Our very own brains are designed to emphasize "we" thinking.
This is our physical purpose on the planet Earth. First to achieve "we" thinking in ourselves and then assist others in reaching it as well. Buddhism serves this well. This doesnt explain the consciousness aspect though.
If our body's purpose is to interact collaberatively with others to further our species - then what is the purpose of our consciousness? It exists aside and independent of our form. It is a 4th dimensional being trapped in a 3-dimensional body that only perceives in 2 dimensions with cues for the third. Getting past these limits is the driving purpose of consciousness. Growth before bodily death and even the cessation of death itself. Ending the cycle so to speak.
Here Buddhism serves us as well. Or at least we hope it does. We have no empirical way of knowing if guiding one another to a specific manner of thought will achieve the lofty goal of dimensional ascension. Either ONE single person breaks through and all of us follow once time is meaningless or ALL must break through together. Theoretically. If it exists at all it is probably somewhere short of either extreme. I view it as a sort of human murmuration (see the Starlings). We communicate with one another because we seek the other's awareness to add to our own. BUT being of similar minds and with homogenization making our needs and somesuch common we should have similar thoughts. To be back to the consciousness whole where our awareness is that of all instead of just our little spot. We seek to be aware as one. Doing that we can abandon other uniform rules that actually hinder us like causation - which makes us think linearly and give time its' power. I know it seems convoluted and complex but it's really simple. We think as one to pierce the veil of of our dimensional bonds. Thats the purpose of consciousness.
This is why we live. To assist and elevate one another to a higher plane both physically and consciously. To improve.
Just to live happily
Even though there are past and future lives , Buddhism is really important to live happily. We all do various things to find happiness . We run after stuff in our entire lives thinking that the end result will make us happy and we get tired all the time. Buddhism teaches us to live in this moment, right now , and how to make this moment happy. Buddhism helps us to make the speed of our thoughts slower . And to get rid of the bad thoughts and make more good thoughts. Ultimately the happiness.
If you can develop your mind to a level of stream enterer , you will feel really happy, most of the time , and you will loose strong attachments , and loose the concept of ‘I ‘ and ‘Me’ to some extent. You will no longer have reasons to cry . You will understand how your mind works and how it changes every moment. And you will be truly free like a bird and be happy all the time .
So that’s just the beginning of the peace . If you achieve nirvana in this life can you imagine how great it will be ? But if you really needed to achieve nirvana , you wont be able to achieve it , because you have to get rid of every need you have , ultimately you have to give up the need to achieve nirvana. Then only you can achieve it .
So basically true(practical ) Buddhism = true happiness. Don’t even have to think about past or future lives . :-)