In the past, I've had the problem with civs no longer offer anything in peace deals once you go to war with any civ for a second time. My thinking was by using a culture bomb to get the other civ to declare on me instead, I could avoid this penalty. I was under the impression that doing a culture bomb was pretty much a guaranteed DoW, but it sounds like this isn't true, at least not all the time.
@swordspider - Is the Citadel something that happens more in multiplayer? I've yet to see the AI build a citadel.