"Life is not that complicated. You get up, you go to work, eat three meals, you take one good shit and you go back to bed. What’s the fucking mystery?!"
Okay, maybe that's the simplified version, but really there's a lot of truth to this. It's missing sex/masturbation to truly be accurate. Actually, the only time I've ever deeply thought about this question is when someone asked me my opinions on it. I suppose there's no 'definitive' answer. Biologically, it's to reproduce and propagate our virus further. Anything else is pure conjecture.