Nice?

This question has bothered me for quite some time and I'm not even sure if it's legitimate or reasonable, or even sane.  But after quite some time, I need to ask it.
"Why are people nice?"  We all know people like this.  They smile all the time.  They greet you warmly.  They always seem cheerful.  They never fail to say hello.  What is going on here?  Are they so friendly because they like to be that way?  Are they friendly because they have been taught to be so?  Are they friendly because they know they are supposed to be?  Do they do it because it makes THEM feel good?  Do they do so because it makes others feel good?
I suppose that the answer could be any of these or a combination of these or something else.  But, to me, it's a ponderous situation, especially since we are told that people tend to be self centered.  Maybe it's cynical to even ask the question or suspect ulterior motives but the question is still there.