^ Well, acting is like any other job or career--if you do it long enough, eventually it gets old. With actors, that means playing the same character year after year after year. On one of his appearances on The Graham Norton Show David Tennant said this was one of the reasons he left the show; he really enjoyed playing The Doctor, and he didn't want the day to come when he would wake up and think, "Oh God, not the Daleks again..."
Clearly, with Eccleston, this was not the case after only one season/series; depending on which story you believe, he left either because he didn't want to become sterotyped, or because he didn't like the environment (i.e., company politics) in which the show was produced.