The fact that I never seem to complete video games. I've been looking through my saved games, I'm nearly done at least 7 games now but stopped on all of them just a few hours before completion.
You know, I've had the same problem. For example, I thoroughly love the "Thief" franchise, yet I had to play through Thief 2 twice before I actually finished it, and in Thief 3 I got all the way to the last mission and stopped. I think I did the same thing with one of the Hitman games, Blood Money, I think. Never even tried to finish Oblivion, nor Skyrim either.
Don't know what it is, but sometimes I have to force myself to complete certain games.