Sometimes I watch very famous deviants (or not so famous) because I love their style and their artwork. However, as time goes on I grow tired of them and their art because they do the same thing over and over again, always the same themes, the same poses, the same styles, the same colours, the same everything. It is tedious, uninspiring and frustrating to see such great artists do the same thing all the time :/ and worst of all, not seeing them improve their skills at all. That's the main reason why I unwatch people.
Also, when the deviant in question turns out to be an ass or a jerk, saying, acting or doing things that I find to be worth facepalming to, I also unwatch them.