It is hard to stick to a game these days. Real life maybe? The overwhelming realisation that our £1k+ PCs are playing titles specifically designed for gaming systems that cost <£200? Who knows!
Batman for example. Loved AA. Love it! But I find myself bored after 30 mins of play of AC. Not that it isn't a better title. It is. But I am just bored.
I think someone else above alluded to the fact it is about WHO you play with. My clan - who shall remain nameless for political reasons - are just a bunch of guys playing together and trying to knife each other. It's super fun! They have just started/stopped playing that Star Wars thingy.
I think the key is to
1. Have a massive HDD
2. Download/install all your game titles
3. Flick between them
4. Get some decent friends to play coop/multiplayer with