Myself, I quit because of coaching politics, chronic injuries (ITBS, knee tendonitis, an inability to lose any more weight) and also because one day I went for a run, finished, and just thought "I don't really enjoy this anymore."
That was 3 years ago as a 3 year college kid. I never got faster than I was in high school (4:06 1500m, 9:03 3000m, 2:01 800), although I ran a 54.6 400 in my second year because I wanted to see how fast I could run one.
I have found myself missing the sport, but looking at the daunting task in front me, it seems like it might not be feasible. I am 30 pounds heavier, I smoke, drink regularly and work 75 hours a week making wicked cash. I don't know if I want to give up the cash just to try a comeback that may or may not work out.
Anyone else been in a situation where you one day you just up and quit, and years later think about starting again?