Why are some football fans so fickle?

Casing point: There was a time when Man U fans were wearing the names of "Hughes" and "Tevez" on the back of their shirts and cheering their names from the stands, and yet now they are roundly booed by those very same fans, and all because they don't like their new employer, what about all the goals they scored and good performances they put in for you? This is all too conveniently forgotten in the fickle world of the football fan.