Why are some people so full of themselves?

In life everyone should feel good about who they are and love themselves. However there are people who think they are above everyone due to looks, money or status. This pertains to both guys and girls. Since I am a guy I will talk from a guys point of view. I notice girls who feel the world revolves around them. They act as if they are the only girl on earth. Some feel as if they are too good to look at. There are girls who look away and hide their face with their hair if they think a guy is staring at them. Most of the times we are not. Just because I am looking in your general direction does not mean I am looking at you. I like girls who are confident about themselves. However when you put others down to make yourself look better, then it is a problem. I feel social media also has a lot to do with it. When some guys and girls get all these likes and followers, it sometimes get to their head. I remember one time I was entering my building and this girl was close behind me. I decided to hold the door for her. She saw me stop and hold the door for her. She then starting walking so slow as if she was cat walking. Playing with her hair and taking her sweet time. There are two doors to my building one that is unlocked and the second you need a key to open. I waited until she opened the first one, I then decided to close the second door right in her face. Yes, it was a douche bag and I am sorry for doing that. I just feel she could have sped up or at least tell me to let it go like most people do. I am not saying all girls and guys are like this, just some. Please don’t bash me for giving my thoughts and opinion. If you disagree or agree please tell me why.