I think it's the same addiction cycle that drives MMO addicts. If work is the only thing in your life going well you may focus on it rather than fix what's broken.

Many successful people are less happey in retirement because that don't get a social circle playing up the importance of what they are doing. Sitting on a beach is simply less rewarding than landing a major contract. Thus, these people are not checking email on vacation because of dedication they are chasing a reward cycle.