Boredom at work is the killer

Well, that and apathy towards the project.

If you're bored with what you're doing and it's what happens half the time, it might be that you'd end up dreading each task that you do.  Or most of it.  For me, boredom is a killer because it's one of the factors that have led me (and my friends) to resign from our former jobs.  If you hear someone getting bored with his/her job and there seems like there's no promotion (or even lateral movement) to look forward to, your friend might be close to the edge to resigning and looking for another job.

As for apathy towards the project one is working on, it could probably be a killer because it's an attitude thing.  The only way for this to not happen is if the person is engaged in the project (not necessarily in a very personal manner) in some way or another.  It's a decision the person has to make.  And at the same time, if you're a project manager, it might be a good idea to find out the source of that apathy etc.