A couple things I never understood about reincarnation. First of all, in about 1000 years, we went from 500,000ish to 7 billion+ people in the world. Where did all those new souls come from?
Second (and this relates to the first question), according to many who believe in reincarnation, if you are a bad person in this life, you get reincarnated as a "lower" animal instead of a human. Then you have to live a good life in order to be "promoted" back to a human in the next life. However, how the heck do you "live a good life" as say, a dog? I never bite my owner's hand? Never piss on the rug? What if I'm a cockroach? Never gross out a human? I just don't get how nonmoral animal souls get graded and promoted back to humans. Seems pretty arbitrary to me.