Yeah, I really can't understand this. I have family and friends who vote differently to me, but I would never let that get in the way of our relationship, and I can't understand the mentality of people who do.
I mean, if someone I knew turned out to be a full on neo-Nazi, then I'd steer well clear. But if someone just votes for the other main party to the one I do, who cares?
Maybe this is the reason why American election campaigns never really end.