No matter how mean/rude/sarcastic someone is they still want the same thing as everyone else: for people to like and care about them. Even if they deny it, it's still human nature. Now, being a bitch is a combination of this (and many other things). It's an armor. And it can get chipped. By people thanking you for doing something. A compliment. A hug. And suddenly the armor is cracked. Then all it takes is one thing to get through that crack and cause pain.
That's why no matter how much I want to hate people, my job, anything, I wish it was easier to keep the armor up.