A question I assume lots and lots of people have asked themselves, yet there doesn't seem to be a definitive answer.
Some people will say it's to be happy; others will say it's to be good; some may say it's to enjoy as much pleasure as we can; and then there are the darker individuals who might say edgy things like it's to hurt others.
As for me, I'm as sure as I can be (which means not sure at all) that we're here simply because we are. We can't control any of our individual cells, nor can we control external events. The only things we can barely (and that's an understatement) control are our emotions and actions on a bodily scale.
Then, at the end of it all, we'll just die. Regardless of whether we go to heaven, hell, Valhalla, or simply disappear, it's a fact that, unless we're either Jesus or Hitler, we'll be forgotten within a couple of generations.
So why are we here then? Does anything we do actually matter? Are we free to do as we please?