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.
You have to understand the time. There was a cold war going on and chess was very important to the Soviets who saw chess as proof that their far-left system of government was better. The Soviets would spare no expense to win at chess and used innumerable armies of analysts to analyze every single move Fischer made from the time he was 15.
Fischer was smart enough to realize this and realized he was facing a Kobayashi Maru of sorts. (ie an unwinnable game). There is no way one man can out-analyze an entire country full of thousands of players analyzing his games.
So, Fischer realized the only solution was to take a break so he could analyze his own lines without other's doing the same. That's what he did and he came back the strongest player in history.
So, after beating Spassky Fischer was faced with a choice. He could play Karpov, who he was undoubtedly better than, and eventually lose the title when the soviet analysts caught up with him.
Or, he could retain the title and ensure that that the Soviets never won the "real" title by beating the proven best player in the world.
The part B of that was that Soviet champions would often dictate unfavorable conditions to challengers. By Fischer standing firm to his conditions, he created an environment where he demonstrated their hypocrisy which paved the way for future non-soviets.
Bottom line, Fischer was playing two chess games. One at the board and the other metaphorical geopolitical chess game. By not playing, he won both. Few people may realize the subtlety of his genius and that is what makes him the GOAT.