Why did Bobby Fischer quit chess after the match with Spassky? I am not asking about the 1975 "resignation" to Karpov, but for which reasons did Bobby Fischer gradually lose interest in competitive chess after becoming World Champion? Jews or not Jews, he did not play a single tournament or match game until the 1992 so-called "re-match".
There is no real definite answer for this question.  Many people have floated different theories, most of which borrow bits and pieces from each other.  It is quite possible that his mental struggles just got the best of him or that he lost interest after attaining the summit of the game.  Psychologically, it must have been hard to cope when you get the thing you've been working for most of your life.  In My Great Predecessors, Volume IV, Garry Kasparov offers a few reasons, arguing that Fischer probably would have lost to Karpov and probably knew it (that point is debatable). 

Regardless, Fischer forfeiting the title was not without precedent in his career.  He dropped out or quit major competitions or chess itself on more than one occasion.  In fact, despite his spell of dominance in the candidate's and title match, his career had almost always been erratic. The reasons for this are debatable as well--I've seen people argue that it was mental illness, strategy, or just intimidation--but the answer is probably a mix of reasons.