Is there a psychological explanation to why some people feel sadness after beating a game?

-I was just wondering why so I and apparently other people feel sorta depressed for a short time after beating a game .

I mean games like Modern Warfare 1 & 2,Fable 3,Mass Effect ect.

These games assuming some of you had played and beaten probably felt type of sad and empty



Why is that?

Shouldn't we feel success or something.



PS:Don't act all mature and tell me to get a life Ok,playing a game is no different than reading a book,it's a whole new world you learn about,there is a storyline and an ending....I believe books are no different....but none on that subject.....



Like i've asked...what is the psuchological explanation on to why sometimes we feel sadness instead of happiness when we beat a long game with a deep storyline?Well, you've been exited and doing your best to beat the game. You obviously enjoy yourself when you're playing a good game, so when it's over, you're sad. No, I have no idea what chemicals in the brain caused this.
its like end of story as movie and when good movie ends ur seen out of the working world that u feel and soon unrealising it u feel sad for a little time