Rin likes gambling against you...He does it for fun, for the thrill, for the knowledge that no matter how hard he tries he still can't beat you...He'll bet small things like a couple of candies just to play some kind of game against you...

He bet his vacation home and lost that to you almost immediately...He likes to use you for reference when gambling with others...He's learning from you and one day he plans on beating you and owning you completely...He's got no problem with keeping you...