I have tried to keep my friends on Facebook who have radically different belief systems from mine, but then my news feed irritates the Hell out of me because rather than post their opinions about something and debate them in an intelligent fashion, they just get lazy, post other people's inflammatory memes, polemics, etc., and rather than hearing a coherent ideology, I get a screed of bullshit. I found myself doing it too, so I've largely stopped posting political things unless I am ready to have a good debate based on that meme that I think would be useful (cf. my recent Columbus Day post on Facebook).

Anyway, I think the fear is the biggest source, combined with the infantilization (juvenalization, take your pick) that comes along with it. However, I would also add laziness, both physical and intellectual, to the mix. If the people with whom I disagreed actually posted thoughtful things, I'd interact more with them.

For the record, I have 2-3 friends from high school whom I haven't stopped following on Facebook who have radically different beliefs from mine. I keep following them because they actually do interact, and try to understand.

I might just make a FB post about this too.