I read that the powers behind West Wing were going to write Lowe out after the first year. It was publicly announced and Lowe was not happy at all. For some reason they changed their minds, but it doesn't sound like an ideal situation and Lowe probably tired of it. He has had a longstanding career with many highs and lows, so I'm sure he will go on in that same fashion and is used to that.
That could very well be true, but from what I understand, he wanted to be paid on a par with Martin Sheen rather than the rest of the cast, and when the show's executives refused, he decided to walk. I personally think the guy is talented, and his character is one of my favorites on the show so I'm bummed that he's leaving, but considering that one of his low points was a sex video with teenagers, he's really pretty lucky that he's on TV at all. Also, since his movie career isn't comprised of hits, and most of the other Brat-Packers slipped off the radar screen years ago, getting paid $70,000 a week to do a hit show that features the best dialogue on television and is nominated for 22 Emmy awards doesn't seem like that bad a gig, even if TPTB did plan to write him out initially. The West Wing has done wonders to restore his credibility, but far fewer actors leave a hit and enjoy subsequent success than vice-versa.
Cheers,
Linda