Back to the original question: there are many reasons and I know people that have covered just about all of them.
Academics: API and Primary ground school are the two mine fields you have to get through before you get near a plane. Anyone with a reasonable ammount of intelligence can learn what's taught but the question is can you learn it all in the time frame given. It's not rocket science, but it's a lot of material in a short time. You have to absorb it. Guys with engineering degrees and the such said API was some of the easiet studying they've done since they were freshman and they have the grades to prove it. However if you're a non-techie or haven't studied a lot in college, you can succeed, but your going to have to put out. Fail a couple events in API or ground school and you'll find yourself standing in front of a board having your personal life dissected. You'll almost always get a second chance but fail some more events and you'll be on your way.
Medical/physiology: Chronic airsickness, vertigo, low G tolerance, sinus/ear problems...the list goes on and on. The flight physical, as throrough as it is, is just an overview in reality. There are dozens of conditions you could potentially have that will go unnoticed until you're flying regularly. The flight surgeons have seen almost everything and they can treat a lot of problems but some things are beyond their control, and the avg. students tolerance, which brings us to...
DOR: Drop On Request. For whatever reasons you've come to the conclusion that flying is no longer something you want to do or that the return is not worth the investment. Maybe the prospect of ten more years in the service is no longer appealing or maybe your family situation is no longer compatible with years of deployments and constant moves. Some people are quick to criticise, labeling them as "quiters" or "losers who can't hack it", but the reality is that it's an agonizing personal decision and unless you're in that person's shoes, you have no idea what you're talking about.