We need no reason or purpose to exist. We exist because the molecules of which we are made of exist, and they assembled into us through natural processes.
As such, we exist because that’s the natural course of things, not because of some higher aspiration or purpose. This holds true not just for us, but for every living entity that exists alongside us, from the most mammoth of beasts to the most minuscule of virii.
However, the most mammoth of beasts and the most minuscule of virii had found themselves a niche to fill. There is no reason why we too can’t find a niche. Let not the absence of a purpose stop us from finding something to do with ourselves. Seek out a niche and fill it. Nature does not grant us a place, but she doesn’t grant anything else a place either. However, the other creatures managed to find their place just fine. Why can’t we?
Seek out your niche and fill it. That’s the natural thing to do.