Let's face it. We're all human, we make mistakes, get in a hurry, and sometimes forget ourselves in the daily rat race. It can be easy to slip up and accidentally cut off another driver, or rush through a door without noticing someone else waiting to go through.
However, people who routinely behave like this are rude and selfish, and I know I don't like it very well when a door slams in my face, or I have to slam on my brakes because someone was in such a big hurry that they just had to get in front of me, only to get stopped right next to me at the next signal.
Sure, there are emergencies, but I seriously doubt all the people I see doing this daily are on their way to deal with any emergency.
I was raised with the "Golden Rule." While I'm not religious by any means, I do believe in ethics and good manners, and that Golden Rule saying of, "Do unto others as you would have them do unto you." It's the most basic lubrication for society at large.
Keeping Your Word
I was raised that, if you promise someone you will do something for them, whether family or a group to which you belong, you bloody well better follow through and do it! If you have a habit of changing your mind, with or without letting people know, you will develop a reputation as an unreliable 'flake.'