Let's face it, not everyone likes their job. (I can get away with saying that because I don't work for this blog.) And what's true in real life is sometimes true in World of Warcraft, as well.
There are plenty of reasons why someone might want to drop one of his or her two primary WoW professions for another. (There might be a reason for dropping a secondary profession, but I can't think of one.) Maybe you were just new to the game and didn't grasp right away that skinning and leatherworking might not be the ideal profession choices for a warrior. Maybe you've just given up hope that a certain profession ever will live up to its potential (*coughengineeringcough*). Or perhaps you got caught up in the min-max-mania that Blizzard is currently trying to eradicate from professions. (At one point in The Burning Crusade, for example, mages with raiding aspirations were very seriously handicapped if they were not Spellfire tailors).