Do you take private matches seriously? Even when it's with your friends? Private matches with friends are supposed to be fun and good practice for the real deal, but some of you tend to take it personally.
Unfortunately for me, I lost a friend today over a simple 4v4 elimination match-up on Skyshock, all because I slapped him the face so hard with an Ice Breaker quickscope. Wish I could have recorded the moment to see the pain he must have felt, because he immediately quit the match, left the party, and removed me from his friends list.
So I ask this. Do you act this way? If you can't compete against your friends, but do decent when playing with them, don't act like a child and quit on them. You should be lucky you even have someone guiding you to victory. I understand the frustration, but you're playing a video game. If you take anything seriously in a video game, I think it's time for you to move on from gaming, or at least play something else (single player games help reduce the stress of "competitive" games).
Play nice, be glad you're able to play private matches at all, and take advantage to hone your skills for Crucible.