I've had a number of campaigns where I didn't lose per se, but I wound up quitting anyway. Usually, however, these aren't campaigns where I'm losing. I actually rather enjoy getting my arse kicked, in a perverse kind of way--it's nice to know that the AI can still beat me.

The campaigns I wind up quitting are the ones where I'm boxed into a corner and can't make any real headway. That's a big reason why I don't play as Aragon and the Danes very often. When I try and play those factions, I usually wind up in a stalemate situation; I'm too weak to expand much, but I'm just strong enough that no one can completely conquer me. So in the end, I wind up just sitting there with 1 or 2 miserable provinces and can't get anything done.