Usually the AI does something incredibly random that fudgs up my entire game flow so I give up.
I am in my groove, my civ is moving along, I'm getting by by the skin of my teeth making friends and allies having a war here or there, things are going great and then... entire planet declares war... or entire planet denounces me... and I know it's going to lead to a giant world war eventually.
I don't play aggressive, I don't build every wonder, I don't have a massive or a tiny army, I don't have a billion cities or stranglehold on every resource, I try to trade and make alliances and so forth but it just doesn't seem to matter. I've only finished 3 games of ciV in 110 played hours because it's just more fun to start again than plod through the inevitable wars against the crap-shoot AI.
Even as I have scaled the difficulty up it's always the same, you can make friends with everyone on the planet but they'll ALL turn on you eventually. They never turn on eachother... it's just... lame...
I enjoy the early game anyway, it seems like eventually you just know how the game will play out so why bother burning the required 60-120 mins just to get there? Boring.
TLDR: Boredom/Disappointment