Sees a civil war in a nation with a different religion

"It's free real estate"

Also can't forget the people who decide to join a secret cult out of sheer boredom.

One of my favorite CK II stories is the time I was playing a random Italian count who heard the voice of Jesus giving him military advice, publically converted to a Christian heresy, then went on a crusade where he met a teacher who convinced him to secretly convert to Islam. That didn't stop him from launching holy wars in North Africa, but I had a plan that once I conquered enough I would publically convert to Islam so that I could turn around and launch holy wars into Italy - unfortunately I didn't realize I needed the DLC for that. But he founded a society for secret Muslims and it wasn't really going anywhere, the meetings were pretty much just him and his son hanging out and shooting the shit, but that's when, out of nowhere, the Holy Roman Emperor himself showed up.

My leader died and my son took over and the game automatically converted him to Christianity because I didn't have the DLC, and I figured that was that, it was fun while it lasted. Then, years later, completely out of the blue, I get a message the the Holy Roman Emperor has decided to publically embrace Islam. In one fell swoop, without a drop of blood, everything from Tunis to Denmark turned green (in the religious mapmode), and the Islamic world, which had been about the weakest I'd ever seen it, suddenly seemed poised to conquer Europe, thanks to the Holy Roman Caliphate.

I quit after that because A) "my work here is done" and B) I'm a cheap bastard and the DLC was expensive.