Why? Because some people are just terrible human beings, and terrible people do terrible things. If you're racking your brain trying to understand it, it just means you're not on of those terrible people.
If you have ever been involved with someone that is rude, aggressive or even abusive; it is always a brain drain trying to reason it out. When you are attacked verbally or emotionally, you spend a great deal of time trying to figure out how someone could talk to you in like that. They do it because they do. The fact is no one has the right to talk to anyone like that. Nor do they have the right to abuse any living thing. It is unacceptable. That doesn't mean it doesn't happen, it just means that you don't have to put up with it.
You are worth more than that. There is no need to figure out why they are like they are, you only need to stay away from them. As it states above, if you are trying to understand it, it just means you're not one of those terrible people.
This is a self-esteem issue for both parties. The one dishing out the abuse as well as the one accepting it. If you really cared about yourself you would never tolerate someone who did not. You deserve someone that loves and respects you, don't settle, don't make room for people that cause you pain or make you feel small.