Why is everyone like this?

Everybody believes themselves to be much nicer and courteous than they really are. People tend to mistreat others regardless of themselves not being liked treated that way, but seldom do people think about this when it's right there. People may do something to another consciously or sub-consciously and may tell themselves it wasn't that bad but when that same action is done to them, they then feel the real brunt of it and become defensive. So why're people like this?