I believe that we live to learn..when we end this life, we move onto another life to learn something else or perhaps the same thing we were supposed to learn in a previous life...all this until we have learned everything we need to know....
I do not believe we come back as animals or bugs. I also think that sometimes we don't move right on to the next life, sometimes we may stay in a transitional state, a spirit form for a period of time.
It is karma. If we fuck up in this life because we are greedy assholes, maybe next time we end up living in poverty, etc. I know alot of people think it's nuts and that believing in some 'heaven' and 'christ' make them feel better, cuz afterall, if you pray and ask forgiveness, you can pretty much live however you want and at the end...you simply say 'please forgive me'. I think that is a christian ideal that lets them get away with anything. I can't live like that, thinking I can live however just to ask some imaginary entity to forgive me so I can go to 'heaven' and live with the rest of the hypocrits....
