I am not worthy.
Why did you stop playing chess?

Nicotine has improved mental abilities in research groups.
I quit when I joined the military, no chance to play or keep up the game.
People around me signed me up for chess tournaments because I was good academically. Never gave me even the briefest of pointers but heavily implied I should do well in Chess because I was "smart". Of course, I could beat a few people but I felt suffocated when I had to play anyone good. Add in a video game addiction (to Real Time Strategy games lol) and Chess was just not that fun. So I quit. Until like a month ago when I randomly picked it up again and with all the awesome internet sources, I am thoroughly enjoying the complexity of chess. We will see how long this lasts.

since you guys want normal patzers too comment...
I wasn't much of a chess player as a kid, but I don't recall deliberately quiting. I wasn't much of a tourney player (ever) but when I had the chance to play some casual game with someone I did.
on the other hand, in this neck of the woods to be a tournament player is a huge commitment of time and effort; and I decided just a couple years ago to make that kind of effort.
Completely or temporary.
My friend once was a master and now his around 1750 FIDE , his 55 now I asked him why did you stop playing chess for such a long time 15 years he told me and he said, because at that time they came up with some regulation rule , that you can't smoke cigarettes anymore and play , so I quit ! I asked why? He explained it to me that cigarettes were helping him think,concentrate, habbit I guess , stress relief etc..
That's one crazy story, please share yours why did you stop playing or took a break.