Why would someone have no friends?

I?ve got a work colleague who to me at least seems like a pretty normal, nice guy. Is he the most fun person in the world? Not necessarily, but doesn?t strike me as a weirdo, is good looking, can hold a conversation, and tries to be friendly with everyone.
I ask him sometimes what he did over the weekend and he used to just come up with a bull**** answer like ?ah man, I was so tired I just chilled all weekend? but I began to notice he said that almost always.
We had a talk this last week and he admitted he doesn?t really have any friends and kind of always been that way.
It was shocking to me because this guy seems totally normal as far as I can tell. So it makes me wonder, how does this happen?


Source.