Of course there's a lot of bad blood between Horde and Alliance, but every once in a while we work together. About half of the times I've done the Ring of Blood quest line in Nagrand I've seen Horde and Alliance fighting side-by-side. There are some other quests that seem to promote this behavior like "Even Gronn have Standards" and "The Skettis Offensive." Some things just bring out our cooperative souls.

Sometimes we just find ourselves compelled to do nice things. Sukalin of EU-Anarchronos told a tale of a complete stranger coming to his rescue:
Just a few minutes ago, I was running down through Stranglethorn Vale on my 20 Warlock, so I could get to Booty Bay and eventually The Barrens to do my Succubus quest. I kept getting attacked by beasts and dying, though.

About a quarter of the way down I was attacked by a tiger and was reduced to about 5% health when a Troll Shaman called Ayisah shocked the tiger and saved me! I was grateful, of course, and expressed my gratitude in the form of emotes - but in addition to that she helped me travel down the rest of the zone by following close behind me on my way to Booty Bay.