Collectible Crap

Why do people collect stuff ?  I heard a guy on American Pickers tell his wife, "I like lookin' at it . . . it makes me happy."   I just keep collecting, but I sell some stuff so other people can enjoy it for awhile too.  Then I look all over the place, trying to find it, because I forgot I sold it, or traded it.  Trading is the best, since I'd rather have something else instead of the money.