OK, so I'm playing nice with a couple of pretty powerful AI's who are asking me often for money to help their war effort. They're both Friendly with me, and not asking for much relative to my economy, so I don't care to hand over some cash now and then.
Suddenly, though, both of them come back at me with the reply, "Way to go, cheese eating surrender monkey!" What a damned insult! After that diplomatic slap in the face, it became my sole priority to take both of them down, not merely conquering their worlds, but doing so in the worst possible ways (gas, core detonations, whatever horrible ways I had at hand), then destroying the colonies utterly.
Better to be a surrender monkey than a bit of space dust!