Until anybody who believes in karma or rebirth fulfils their burden of proof and persuades me, I'm not going to live as though they're true. Why? Because I have actual problems to solve in my actual life, and I can't do this unless I'm only factoring in things which are likely to be true. Considering that the tools of dharma practice that Buddha laid out deal with actual problems for my actual life, I see no reason to distract myself by clinging to past lives or yearning for future ones. I see no reason to worry about them at all. Aren't we supposed to be living in the present and aware of what's going on around us now?