Why do we exist?

From The Matrix Reloaded:

The Oracle: Candy?
Neo: D'you already know if I'm going to take it?
The Oracle: Wouldn't be much of an Oracle if I didn't.
Neo: But if you already know, how can I make a choice?
The Oracle: Because you didn't come here to make the choice, you've already made it. You're here to try to understand why you made it. I thought you'd have figured that out by now.
Neo: Why are you here?
The Oracle: Same reason -- I love candy.

In other words, all of your actions in the material world are determined by psychological forces, by physical forces, etc. But that doesn't make life meaningless. On the contrary, you are not here to make decisions. Instead, you are here to meditate on the choices that you make. Life is a sermon for your edification, a koan to aid in your enlightenment.

And God is in the same boat as you. God is being edified by the choices that we, individually and collectively, make. He loves candy -- God simply takes pleasure in experiencing with us how existence is manifested through time. We exist so that God can savor how it feels to be.