A lot of people are without original thought. They act as they see others acting. People want to be right and they want to be accepted as equals. This often leads to them being shallow, and tiresome, lame, unoriginal, etc. You might call this conditioning. Man is evil at his core and shouldn't be trusted. I know people like this and I am not one of them. Most people are this way though, unfortunately. They all want to be the same shitty person I suppose.
I would have to say, in all seriousness, that crap-itude is in the eye of the beholder. To themselves, they are a laugh riot. They are the funniest thing that has ever come out of diapers (which an appreciable percentage of them may, in fact, have done). The root causes of their self-absorption and lack of perspective ... a derth of mirrors in their homes.
I think a lot of people love the sound of their own voice and have truly never learned the art of listening. This "selfishness" is becoming more prevalent every day.