As an example, I've let one of my very best friends drive my car several times in an enduro in order to keep his license current (plus, we all have a good time). I used to get it back with severely flat spotted tires (to the cord). I let that go as they would be used up during the race anyway. Three years ago I watched him make a stupid passing attempt that destroyed my spoiler. I built another one and said nothing. Last year, he T-boned another car and ruined my bumper (lucky that was all). I repaired that, too, but I've bitched about it constantly since. This year he failed to see a climbing temp guage during a full course yellow and the motor is now a boat anchor...with the accompanying DNF). Not only do I have that to repair and replace, but he still owes me for his portion of the expenses. Now, as I said, this guy is one of my very best friends in my life. He and I would do anything, anytime for one another and our families. But, in order to save that friendship, he is no longer going to drive my car...anytime, anywhere.
Think about what you get into when you share something that means so much to you. All the incidents with my co driver could've been avoided, but also were racing incidents that could've been blamed on other circumstances. But his friendship is more important to me than racing.

Just my two cents.