Sometimes or they act like they don't understand and you're the bad guy when you treat them like they treated you all along. It was no big deal while they were doing it (because "that's just how they are" or "they were just trying to make so and so happy"), but you're just mean when give them they same treatment that they have given you the entire time you've known them!
Get your head on right. If you're having a bad day, stop whining about it and start taking action. First--adjust your attitude. Decide to be happy, not sad. Decide to be powerful, not weak. Decide to be part of the solution, not part of the problem. The shortest distance between a crap day and a great day is a decision to adjust your attitude. One decision. Works every time.