People who get way too competitive have ruined games for me. There's a reason why I don't care for multiplayer games and I'm "eh" on doing stuff like board games. Far too many people just can't handle winning OR losing and all of a sudden I have someone that supposedly loves me on some level in my face telling me how much better they are or how I'm a dick because I happen to be lucky enough (yes people, a lot of group games rely on some element of luck usually) to be winning/have won.

Life in it self is stressful enough. Don't bring that negative garbage into a situation where people should be relaxing and bonding.