There seems to be this ongoing debate both within the Buddhist Tradition itself and with the world-at-large as to how a practitioner should relate to the world. The Buddha himself forsook his family and all worldly-pleasures in pursuit of a spiritual life prior to his enlightenment. Such practice and attitudes has largely been preserved within the Theravada tradition. The Mahayana, in particular its Chinese strain, stressed on the ability to heal the world within and without, to relate to all sentient-beings in a loving way through the practice of Metta and Karuna - the Bodhisattva way of life. Compared to the Christian Gospel of love, the Buddhist doctrine seems so much more rational and pragmatic - Metta is just a means to an end, not an end in itself; it is practiced just to help the yogi remove obstacles from his or her road to nirvana. It is not uncommon to find Buddhists who are aloof to worldly affairs and loving relationships - the yogi who chose to lead a life of solitude and inner-peace, in place of worldly pursuits; the vagabond who chose to wander in this world so that he may find his true home in another. My question would be: How should an aspiring Buddhist aim to lead his life and relate to the world?
From my own experience, at certain point in one's practice, one's ties with the regular world reduce so much that one no longer gets any energy from participating in mundane affairs ("There is nothing further for the sake of this world").
At this point one has only two choices: either go 100% hardcore ascetic ("Arhat") or engage with society in the teaching/helping capacity ("Bodhisattva") - either openly or secretly. One can't really be "normal" anymore, pursuing success and entertainment, since one's value system no longer goes up to worldly goals. So if one were to engage with the world lukewarmely on samsaric terms - then one would either die miserable death, or regress badly -- hence only two reasonable choices. I suppose this is why it is often said that once you go far enough there is no turning back.
As to which option is preferable, I think it's totally up to personal inclinations. Some people are very self-sufficient and very comfortable being the end in themselves. For them the peace of minimalism is way more agreeable than the high drama of Bodhisattva's path. Other people need to get energy from the outside, so they chose to give something, to get something in return.
The one who wants to become a Buddhist should take refuge in the Triple-gem. It means taking spiritual guidance only from the Buddha, Dhamma and the Sangha.
If you are a lay person, you are expected to put forth and honest effort to keep to the Five Precepts on a daily basis. Eight precepts are recommended on full moon days. Lay Buddhists are allowed to work for worldly ambitions such as running a business, getting married, having a family etc. as long as they do not do it in unwholesome ways.
If one wishes to dedicate one's whole life to Buddhism, one may renounce the lay life and get ordained as a monk or nun.
How should an aspiring Buddhist aim to lead his life and relate to the world?
Imo Buddhism do not tell you how you 'should' live your life. Other people tell you how you should live your life. Buddhism will tell you what sort of lifestyles is wholesome and what is not. But whether to follow it or not is up to you. Because at the end of the day what you do is all on you.
Having said that, Buddhas analogy of a lotus flower, which is born from muddy water, fed from muddy water, but risen up from muddy water toward the sun and living untouched by muddy water describe the mindset of an enlightened Buddhist.
You are born out of the world, you are fed (depend on for your survival) by the world, but you rise from it and live untouched by the all the unsatisfactory things going on in the world.
I would expect that further you travel on your path less you will be attached to the world.
The Middle Way is not an extreme in itself. Mahayana swings towards concentrating on benefiting other first (more anatta-y), Theravada swings towards concentrating on benefiting oneself first (more anicca-y). In the end, whilst 'defining himself' as a meditator at heart, the Buddha never declined to answer an honest question on the Dhamma. It is important to live in line with all aspects of the Noble Eightfold Path.
The world as a conditioned, dependently arisen experience, is seen as a burden. In stating an accurate and verifiable description of experience within anicca, anatta and dukkha, curiosity for the world becomes less of a driving force, so engage with the world out of compassion.
"What is one's relationship with the world upon attaining Nirvana"
The is no more relation to the world (in all its meaning), upon attaining the Unbound (Nibbana).
If asking "is there any relation left", one could say "formal to the Noble Ones", if spoken of kindship.
Yet, if asking "what is the benefit, the use, for others of that?" Being beyond of trade and exchange, Such is an unexcelled field of merits. Directed to the Unbound as well.
"How should an aspiring Buddhist aim to lead his life and relate to the world?"
In using food/relation/sustenance/entertainment just to abandon food/relation/sustenance/entertainment.
"How do we work to get there?"
By working on the conditions for the capacity to practicing this Noble Eightfold Path
[Note: This is a gift of Dhamma and not meant for commercial purpose or other low wordily gain by trade and exchange]
The [Tibetan] Mahayana tradition teaches that nirvana is a direct understanding of emptiness. This direct understanding of emptiness is gained by a combination of study and meditation on emptiness. This direct understanding of emptiness is also a direct understanding of reality and liberation from samsara so it becomes obvious what is important, and what is not. For example, generosity, patience, compassion, kindness, etc. I have been taught that the Bodhisattva way of life is required for full enlightenment. The ultimate goal is to become fully enlightened in order to be of the most help to all sentient beings.