To the first part, there isn't a god. To the second, [MENTION=10496]skylights[/MENTION] nailed it. It's simple biology. Pain is an evolved warning signal that our well-being is being compromised. Life is survival for its own sake. Eating, drinking, growing, healing, fucking and reproducing, it's all about continuation. Life is painful because life continues by design. If it did not hurt, we wouldn't exist at all, because we wouldn't bother avoiding those things that hurt us, and we'd have all died off long before we even became human, instead of just those who couldn't figure out how to stay away from the things that hurt.
Asking why we don't just not exist to avoid pain is problematic. None of us were given a choice whether to come into being. We were shat out, against our will, by our mothers years ago. Those who think that's too much of an indignity give up and die, and those who like the good parts fight on to continue to enjoy those good parts and dance around the bad. Obviously, life is really fucking hard at times, but it's pretty great to be alive.