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.
He had mental problems. But he is still one of the greatest chess players of all time.
Joseph Ponterotto has even written a book about Fischer's mental problems - A Psychobiography of Bobby Fischer
Ponterotto believes the evidence is strongest for paranoid personality disorder, a psychiatric condition characterized by unrelenting paranoia and suspicion of others, but is not schizophrenia.
Fischer declared in several media channels that his goal, all of his life, had been to become World Chess Champion. Once he had accomplished that, he concluded that playing chess competitively was no longer a challenge, and that if he did not have something significant to gain, there was no longer any reason for him to play.
i met fischer and talked to him and know people who played chess with him and had closer interaction
imho he had some social adjustment issues and believed that chess players should make more money and that he should improve the game and circumstances but was frustrated by the bureaucracy