I always try to beat my games. I only don't if I really don't like the game. There's been a couple of handfuls of those.
Completion is another story. If I really enjoy a game, I'll try to go for 100%. Or if going 100% isn't a hassle where you don't really have to go too far "off the path".
And then achievements. Bleck. I try to go for them "naturally" as much as possible. If, once I'm done a game, I'm only missing like... two or three, then I'll try to get the rest (this was the case with Assassin's Creed II). But the idea of achievements just bugs me. I don't need extra points for beating a level I'm supposed to beat and I don't want to stand in one spot and shoot the same gun repeatedly for five hours for extra points.
I play games purely for fun and entertainment. If I enjoy the game I'll want to to play more of it - either replay it or complete it. If I don't, I'll leave at it beaten or even don't finish it all. I don't need bragging rights or to grow my e-penis.
I don't think I can get distracted easily. As I've gotten older, I ended up with less time on my hands. But when I younger, I could play a few games to the end in a single sitting. I didn't do it often, but I did it with Banjo-Kazooie, Paper Mario, Ocarina of Time, and Super Mario 64.