According to the boy himself, he left TNG to focus on his film career (outstanding move). And he simultaneously cut ties with everyone in the cast, people who were by all accounts dear friends. Why?? I can't recall the specifics and can't find the piece I read years ago about it, but I think it was this thing where he resented the fact that everyone thought of him as Wesley crusher and was afraid of being only that forever. So he left the show, flipped the bird to his friends and didn't come crawling back until, decades later, being Wesley crusher was the only thing he was liked for.