Why would a male friend of mine do this?

He used to try to "taste" his good friend's sundaes..when they asked "would you like to taste my dessert?" He not only did this,but, he also began to eat more and more of it..and could NOT stop!!! I think that this is pretty rude! Even for a grown man!!! What in the world is happening to him? I have told others (our mutual friends) to NEVER ask him to try or taste our desserts..because he takes advantage of this each time!!! He has even taken his own bowl and licked it in a restaurant!! Good Grief!!!