If you play a particular game a lot, you're bound to get good at it after a while. If you get really good, you may start to find patterns that make gameplay easier. If you refine these patterns, you could become a master. If you've become a master, you may start to see redundancy of experience, where, say, leveling alt after alt becomes repetitive and humdrum. If this happens, you may start to look for the truly novel places, the unique situations, the unusual character builds -- you may quest for something specific.
Sometimes, it's that ridiculously rare piece of armor that only drops once every thousand encounters. Sometimes it's that spell waaaay down on the ability tree that requires an exhaustive amount of time to unlock. Sometimes it's an in-game event that occurs randomly, when you've collected the proper sequence of items -- and you don't know exactly what those items are, but you saw someone do it once, and you're damned if he's going to be the only one. When you're so good at a game that the standard challenges no longer entice, do you go looking for something specific?