Pure dopemine.
Almost no real gameplay other than moving around. The real reason why it's so much fun is due to the absurd combos and builds you can make and the progression.
Just when you think you've seen everything this game has a new update comes out with an entirely unique new thing to the game and then you're back in and playing for another hour or two!
This game is pure crack and I can't get away from it. Not that I'd even want to :)