I know I should read the whole thread before posting, but I I just now saw this thread and don't have much time.
I recently left a band that I had been with for seven years. It wasn't until after the fifth year when we got a promoter (our guitar player's girlfriend), and we got insanely busy. As soon as we started playing heavily is when I started getting unhappy with pretty much everything about being in that band. You wouldn't have known it when I was playing; I only brought up my complaints behind closed doors with the band members. Finally, a little over a year ago, I had a big argument with the band in a meeting and told them that if they wouldn't slow down that I would be leaving. They said they would slow down, but they didn't, so I announced that I was leaving. My breakup with them was a long time in the making, but to all of the fans it looked like a very abrupt decision, and I had all kinds of people hounding me about leaving, and between the fans nagging me and my drummer calling me every other day trying to talk me out of leaving, I got really, really sick of hearing and talking about it. And that was just a local band. I can't even imagine what KM went through when he announced that he was leaving DT. Since he's a professional musician, I'm sure he too was capable of looking like he was having a good time whilst playing; for that matter just because he wasn't happy with the whole situation doesn't mean that there weren't pleasurable moments on tour that he still cherishes.
His reasons for leaving are not the same reasons I left my band, but I can relate to how he must have felt while making his decisions, and maybe he had to put up with a lot of negativity coming from lots of directions and it pushed him to the point of not ever wanting to talk about it with anyone.
Just my two cents.