Whatever gave you the idea that that question is easy? It's probably one of the hardest questions in existence. People make full-time careers out of figuring it out, and so far the answer hasn't been found (and it never will, but don't tell them that).
This is probably the opposite of what most people will tell you, but my advice is: don't think about it too deeply. It doesn't work.
There are lots of opinions and philosophic theories about that. My point of view includes:
- to find your partner, your soulmate to create a family with. To have children. To love each other. " The happiness only real when shared";
- to enjoy - to get good emotions, to eat tasty food, to have fun and so on.