Supereor: ...and I can't get into any of them.
I think sometimes if you play games in the same genre one after the other you may "game out", as avidly noted by HereForTheBeer and Rusty_Gunn.
For example, if the last game you beat was an epic RPG, try a shooter, a strategy game, a platformer or a racing game afterwards. That happens for me from time to time - you get saturated by similar-type gameplay.
Specific tips for some games that I'm familiar with:
Morrowind/Oblivion/Skyrim/Fallout3: try not to stray too far away from the main storyline. Sometimes you may find yourself bogged down by "bring-this, kill-those-guys" type of side quests which may sink up lots of time, and also make you too powerful so that main quest bits feel a bit easy after that.
Fallout: New Vegas: The opposite: do try to get into the side quests - some are good fun! Also, don't neglect companions - their personal quests are great too.
Doom 1+2/Unreal Tournament/Quake 1-3: Weekend+ a LAN game with friends+ lots of beer=now you're getting into it :)
Super Meat Boy: hell, I can't get into it too! The time has not yet come, I guess.
Finally, take a break from gaming. When you come back, you'll be hungry for whatever :)