Looking at human relationship from an economic point of view

I read the post on our AP economics blog that compares friendship with consumer mindset and entrepreneurship. I found this interesting and started to think about how human nature, as we perceive it, would be used in economics and what the limit of it is, because I did find the concept of connecting the motive to form a friendship and the nature of economics, or I call it greed, odd at the beginning. Furthermore, the blogpost points out that a virtuous, true relationship goes beyond the scope of economics, meaning that forming such kind of friendship would be considered 'irrational'. Is the economics theory here right? Or is the assumption of human nature that the economic theories are based upon missing something?  And to link all these questions to myself, is rationality really 'better'? or 'fairer'. I am aware that I have been rambling a bit, and I do think I need to take some time to think this through.