Did you ever wonder why people are so rude and don't consider others?

An example was today I was at a restaurant. There was only me and one other couple in there. The couple was setting with their meal while I ordered my meal. Once I got my meal I want to condiment section was was getting napkins and sauce. The woman got up and squeezed right in between me and the condiment table getting stuff too. She had plenty of time to get that stuff before I was there. And I was there before her. I just moved back and let her do her thing. There as no anger or malice in her actions. She just pushed ahead to totally oblivious at how rude she was being.