Sounds like you have Obsessive Completion Disorder. things like achievements hunting or looking for all the 100 special items are just awful. You should play something for fun and because it is cool. As soon as you go for a 100% completion or walk around for hours to look for shit, you really have too much time on your hands.
Then you're not what psychologists call an acheiver. For some people, goals
are fun. Both the effort itself
and overcoming the difficulty are the things acheivers are looking for. If you only cared about fun, you'd be playing flash "break your mouse" games 24/7. In a game that lacks goals, many people will create them.
Think about it : do you live only because you want to enjoy drinking, eating and having sex ? No. You do care about friends, morals, doing great things etc. That's exactly the same thing. Humans aren't like water, always going down the slope. They sometimes go up because even if it's more difficult, it's a great feeling to be up there.