When my armies are growing, when my income is rising, and I see how insignificant and pitiful my neighbours are becoming compared with me.... When I am able to hire lots and lots and lots of mercenaries and I just have to have a need for them... can't let al these Khwarazmian Cavalry units, Highland Clansmen, Jinettes etc. just sit there doing nothing, eh?
.... When I help Spain geeting rid of the Almohads and the Spanish then treacherously invade my province, ungrateful dogs!
....When there's a fleet of another faction sitting next to my fleet in a sea region. Sorry, it just looks ugly and annoying. I just have to send that fleet to the sharks.
......When there's just too much Priests, Alims and Inquisitors running around in my provinces. I have to kill kill kill all of them!
.... When I see my enemy's empire is becoming bigger than mine! I send in a enormous army of spies to split that empire up in a chaos of rebellions, and then invading what's left!

I can go on and on, I think. But I am hungry and I need to buy food and do homework. I do have a life you know? Don't you?