I don't play on a pvp server for the sole purpose of being able to gank people. I almost always /cheer at them and help them kill whatever they're fighting. When I run into Horde players, I want it to mean something. On a pve server, if you run into a player of the opposing faction they just ignore you because there's very little interaction possible. On a pvp server, even if you just /wave, /hug, or help them kill a mob or few, it means something more because you actually chose to help them when there was an alternative.

Since there's no alternative on a pve server, it doesn't mean as much when you're friendly to the opposing faction. Being nice to somebody ceases to be interesting if it's forced upon you as the only alternative.