Believing in Karma is like learning the Golden Rule. It's just what we learn as kids, it's considered truth and we're not to question it, we're just to abide by it. Period, end of story.
Yes, this is all well and good. I leave a good portion of my life up to Karma. (I know for a 100% fact that a couple of very bad things have happened to me as a result of bad Karma juju I emitted to the world). Mostly, though, I believe in the Karma of kindness.