If I see a weakness, I attack. For example my very first real multiplayer game. The second to best scoring player was the weakest as I saw it. We just had opened borders, and his defenses...pathetic. I had myself a good combo of spearmen, axemen, and swordsmen coming up to him. I would have gotten horses too! His cities were defended with one archer a peice. The thing is, the point of the game is to win. At one point you can be best friends, At another point you can be enemies. Take single player...Ive stabbed plenty of AIs in the back, and Im sure you have too.
Sadly the dude left the game a few turns before I declared war ;(
Then I declare war on everyone else...and they started whining and left...SISSIES >.<