I think his reason for leaving was because it was becoming too stressful but I also believe he started to think he was more important than Morrissey. I think he started thinking the whispers were true. I've had conversations with people who say Morrissey would be nothing without Johnny Marr whereas I think the exact opposite. Mark Simpson in Saint Morrissey, and I'm paraphrasing, said such was the magic of Moz that he was pulling the universe towards himself and he would have become an icon regardless of who he collaborated with. Don't get me wrong Johnny would have forged a good career due to his supernatural skills but really that is what he forged after The Smiths. Morrissey went on to greater heights but Johnny certainly didn't. There's no denying Morrissey had a fair few moments in his solo career that were of equal cultural significance as The Smiths whereas Johnny got nowhere near.