I went to a Senior Military College (VMI). Nonetheless, I think it's similar enough of an experience to a service academy that I might speak somewhat intelligently on the question.
I had every intention to make the Air Force my career when I started as a Rat at VMI. I left the service after my ROTC committment ended.
Why?
Mostly for the reasons people on here have already stated. Essentially, I wanted more control over things in my life. You do give up some freedoms for the good of the service while on active duty, and I wasn't keen to move every few years and stay in a career field that wasn't satisfying to me. I was stationed in Minot, ND, and had I remained in the service, I would have had at least another year there before the ability to PCS to such a garden spot as Great Falls, MT (which, compared to Minot, is like Paris). I was in ICBM operations, and the mission, while important, is one giant exercise in waiting for an event which, thankfully, will probably never happen. Because of the nature of nuclear operations, the standards are very high and demanding (e.g. a 98% on an emergency war order test will have your Squadron Commander bringing you into the office on an off day to "study EWO"). That can wear on you after a bit. Being on the Personnel Reliability Program (PRP), which means you have to go to the doctor for every sniffle (can't take NyQuil on your own when you feel like crap), can also be tiring.
I realize a lot of guys have had it really tough over the last few years with deployments overseas, and probably, my gripes sound a little wimpy in comparison. Nonetheless, there are other aspects of being an officer in any career field which can be draining. Simply, I was just tired of the Air Force, and I wanted to do something else. Since then, I used my GI Bill to go to law school and moved back to where I'm from on the East Coast. I got married and have a good life with a steady job that doesn't dominate that life. I don't regret my time in the Air Force at all. In fact, I treasure it. Still, leaving when my committment expired was the best choice I've ever made.
There are as many reasons for leaving the service as there are people separating.