Short Answer:
Extraordinary People: The Boy Who Lived Before Long answer: In general, I have problems with believing in invisible forces (like the modern belief of how karma operates.. though I'll mention later what explained it for me) and almighty beings.. Not that I think these invisible forces and/or almighty beings can't exist, just that it doesn't make a whole lot of sense to me. So therefore, I can't believe in it.
I was raised around the eastern beliefs of karma, reincarnation, and enlightenment. I was going to write this whole paragraph about how enlightenment makes perfect sense even from a plain (non magical, "everything that exists we can see with our own eyes") POV, but fuck, my hands hurt, so you guys just got saved a whole big schpeel. So anyway, Gods don't make much sense to me because I wonder about where they came from, what they do all day, the point for their existence, etc.. Before I better understood a plain POV of karma, I began questioning it after a while because it didn't make sense to me that there could exist this invisible force of basically checks and balances that would shape my fate - essentially - so that if I pissed on my neighbor's door (and believe me, they're rude and ignorant.. they deserve it!), someone may shit on mine... or maybe I'll fall and bust my lip. All because I didn't smile and take it. Yeah, doesn't make a lick of sense to me because there must be something behind this, and of course I can't wrap my head around that.. but it was Depak Chopra who coincidentally explained karma in a way I could get it. If I punch someone for being an asshole, that affects me mentally. It affects how I choose to view the world. This continued mental philosophy of course will have an effect on my life since my actions will affect those around me and how they treat me. Okay, that I can get.
Unfortunately, I don't think reincarnation can be explained in a non-metaphysical way. But, this I am willing to accept in some ways because I think there are a lot of things that we regard as mystical and unexplainable because we just don't have the technology to explain or understand it. Like acupuncture, for example. I've documented a number of case studies that show it works and it's not a placebo effect (even though *cough* ignorant dickweeds that swear their allegiance to modern medicine - a system that is documented to be the #1 killer in America at worst, and at best, responsible for 100,000 avoidable deaths anually - will insist up and down that all studies show Acupuncture isn't better than a placebo (hint: they don't read the case studies that come out) *cough*) and acupuncture is supposed to work by directing the flow of energy to fix imbalances. It doesn't happen always, but most times you can feel the energy (Qi) when it's done (and no, it's not my imagination. My imagination isn't so strong that I can feel an effect I'm not even expecting) moving... most of the time it's not very strong, but a few times it has been very strong. The point I'm getting at, is that: acupuncture works even though science can't explain why and when you undergo acupuncture treatment, you can feel the Qi move (this was a surprise for me since in the books I've read about acupuncture, none ever mentioned this).
My father was able to do age-regression (a hypnosis type of psychotherapy used to take people to an earlier part of their lives to figure out the cause of a certain emotional problem) to bring people into - what he believed I guess - was a past life. Coincidentally, I never did it with him.... I really can't explain why, but I just never did. Jenna did however, and she saw something that makes perfect sense to everyone who knows me well: she saw her and I drinking tea together as women. Of course, that's not very conclusive; it can easily be argued that she saw this because that's what she was expecting to see. But, the stunning thing is, unbeknownts to many, there have been a lot of children that have expressed past lives and details about these lives that they could not possibly have known at such a young age. There was a preview for a documentary I watched at Integrative Nutrition, but I forget what it was called. So for an example of what I'm talking about, see
this documentary.
Of course, the existence of reincarnation - I feel - doesn't affect religion any. Some religions mention it, and some don't. I think that most religions are defined by the period they were born in and those who wrote about them. I think that most religions profess the same thing, just in a different flavor. I only say this, because some people feel that if something like reincarnation is real, that it somehow weakens their faith since it's something not explained - or even disbelieved - by their holy scriptures.
ps: I'm too lazy at the moment to add citations for the medical stuff I ranted about. They are of course available upon request since I stand by my word that I can prove everything I talk about.