You got "existence is suffering" right. Some people think "life is suffering", which is the wrong interpretation. Life as you say, is a roller coaster of many emotions and experiences.
In one's neverending existence, one is subject to birth, death, rebirth, experiencing endless ups and downs of pleasure and pain, joy and sorrow, gaining and losing loved ones, having unfulfilled desires, experience good and bad health etc. Nothing that is joyful lasts forever. Nothing that is sorrowful can be avoided forever. Just imagine undergoing these for eternity without rest. Not just as a human, but also in other realms. This is the first noble truth.
Of course, you are not compelled to try to escape this "suffering". If you enjoy your current state of existence and life, you should really continue with it. There's absolutely nothing wrong with that. There are many who come to Buddhism, for reasons other than Nirvana, like seeking peace of mind through the meditation techniques, even if it's only for a short time.
The second noble truth states that this suffering is caused by craving to what is satisfying and aversion to what isn't satisfying. Of course, one type of craving is to sensual pleasures that doesn't require explanation.
Another is the craving for "becoming", meaning, the craving to be a person, to live a life, to thrive, to seek happiness, to build a career, to raise a family, to be your own man or woman, to experience all that life has to offer, to have a personal history that is worth looking back to and telling to your grandchildren. But unfortunately, this will not last forever, and everything that you have built will pass away, just as you would. Family members could turn against you, wealth can suddenly diminish, careers can crumble, things that you desire can be taken away from you etc.
Then this leads to the third noble truth. That suffering can be ended, by removing this craving and clinging. When suffering is ended, Nirvana is reached. This source defines Nirvana in a number of ways including "the stable, the peaceful, the deathless, the sublime, the auspicious, the secure, the wonderful, the amazing, the unailing, the unafflicted, purity, freedom, the island, the shelter, the asylum, the refuge, the destination." In other words, the goal of Buddhism is to provide everlasting bliss in Nirvana.
But what does this mean to ordinary beings who are far away from the destination and refuge of Nirvana? Buddha wants us to seek joy, but not in things that are impermanent, because things that are impermanent will not provide you with that stable joy that you seek. But does this mean that we should avoid life and avoid pleasures?
This then leads to the fourth noble truth, that the path to Nirvana is through the Noble Eightfold Path, that supports a balanced life. So, that you may still live a life, seek happiness, raise a family, build a career, thrive, enjoy pleasures etc. but within a framework that will eventually lead you towards Nirvana, rather than spiralling towards the perpetuation of suffering.
Buddha also taught the practice of the Brahmaviharas. The first three of these are loving kindness (metta), compassion (karuna) and empathetic joy (taking delight in others' well being). So, Buddhists are not meant to be dead serious. They are meant to radiate with joy.