Maybe Grossman is like a gorilla, and he believes showing his teeth is a sign of aggression towards others? Though if that were the case then surely there would be pictures of him showing his teeth while throwing a check?
Hey, I'm not a dentist, but that could be a mouth guard in that picture too, DaStars99. Now it would stand to reason if he wore a mouth guard it would be to protect his upper teeth, but maybe it's all a ruse? Maybe he's trying to give the impression that he's got a full set of incisors and bicuspids, looking for that all important (insert Stars official dentist here) endorsement?