its amazing how serious people get in the US about which party they support. maybe its just my family here, but my parents couldn't care less which party i voted for, unless i voted BNP or something. we have a discussion about it when the election time comes, but its not like we voted labor and we will always vote labor! i have an opposing view on politics to most of my family as well, we have a small discussion over tea about it sometimes, but thats about it. never fell out with anyone over it.
main reason i can think of is there a lot of difference between the different US parties, where as in the UK the labor and conservative party are not really that different. our conservative party will still be considered left wing in the US.
Actually, I'd guess it's the opposite. Here in the U.S. there are very few real differences between the parties, so people who support one or the other feel compelled to make a show of it, almost as if they're trying to convince themselves that they are distinct, and that their vote does matter. OTOH, from what I understand about, say, Germany or Sweden, which government you vote for will have actual, noticeable, real-world effects.