Difficult people are everywhere, and you do not have to rationalize their behavior or consider their motives to deal with them (or 'un-deal' with them if that be the case). While it is true that quite often difficult people themselves do not know why they act the way they do, what is much more important is that you get in charge of the way that you react to them.
The biggest problem with difficult people, whether they snap at you, are demeaning, or are just plain rude, is to never engage yourself in any feelings of responsibility for that person's behavior. That's a rather detached way of saying, 'don't take it personally.' Difficult people may trigger feelings of guilt, resentment, and defensiveness. You may feel that your rights have been violated. It's easy to get smug and think, 'How dare he treat me like that'? If somebody really burns you, you may even fantasize ways of revenge (or regret that you were wimpy and didn't 'show them' with that snappy comeback that came to you in hindsight).
The truth is that you can't control difficult people and even if you could, what would you do? Get them fired perhaps? Would you really feel good about that? Besides, he or she would just move on to the next place and be difficult there. While there are downright sadistic people out there for whatever reason, people are in general testy because they are not very happy. So actually, when dealing with difficult people, it is in your best interest to say something kind to them and mean it from your heart. Think of the next person who has to deal with him after all. You'd be doing him a favor too if you could lighten up the grouch a bit. So when leaving, look him squarely in the eye, smile, and say something like, 'I hope you have a really great day.' And don't be sarcastic; really mean it. People can tell.
