I think it's pretty clear why they broke up. He even says, I was acting. Any artist should know that when they start phoning it in, it's close to the end.
I get it, and I'm glad they decided its time to quit before they do something to drag down their reputation.
This. I don't know if everybody wants a dramatic reality show moment here or what. He clearly gave a reason. And an awesomely well-written letter. Props to him for stopping when he lost the love for it. 12 years is a pretty good run.