What if there is no “why” behind anything? What if there was no [heaven/nirvana/god/enlightenment/etc./etc.] and this was it? To me, it seems the sensible thing to do in that case would be to enjoy the present moment, apply myself so that I experience fulfillment, and be nice to others.
But if you get down to the core teachings of all the “why” propagating beliefs, isn’t that basically what they tell you to do? 😉