This is a very vague question, but possibly one of the most brilliant, to which there are many possible answers. We are, presumably, discussing the Meaning of Life, are we not? For example, Christians would argue that the purpose of our existence is to love God and each other, and there are infinite other reasons for us to speculate our existence.
My personal favourite is the only one that to me is the only one that makes logical sense. It would be of course that there is no reason four our existence; we are merely the end result, so far, of one possibly completely accidental chemical reaction, resulting in the Big Bang, and then the universe, and then, through an extreme fluke of solar parking, conditions on Earth that allowed life to evolve, and bring us everything we have today, and everything we still have to come. Embrace it, I say, and love every minute of it. If it turns out that one of us was right all along, then there we are.
I'm not saying I'm right, it's just one theory.
"This is the worst day of my life."
"Correction, the worst day of your life so far"
- Homer Simpson