Some people can't take criticism even if it's constructive. I was doing a LLDF with my Monk and the tank would mark and then lose aggro on that marked target as well as everything else charging the healer. After we wiped, I asked him....

Me: "Are you flashing.?"
Him: "Yeah, LOL, I had all the aggro.. that's why I died first."
Me: "No, you died because the healer died. Everything is running all over the place. Just flash a 3 or 4 times to start and it should be fine."
Him: "It's just a lowbie dungeon, who cares?"
Me: "I'm not mad, haha. I have both tanks I90 and was just giving advice."

And then he dropped.