He's had to leave people behind too - it feels like betrayal and unworthiness and shame - so he knows how much it's hurting this brilliant young man. He repeats over and over that it wasn't your fault, Jim, there was nothing you could do, they understand why you had to do it, it wasn't your fault, it wasn't, I forgive you, even though he knows from experience that no matter how many times he says it, it won't make a bit of difference to the captain who so badly needs to hear the words.

This is the cruelest thing he can do, but it's also the only think that can work, so he adopts a commanding tone to snap the kid out of it, "Captain Kirk, as your Admiral I am ordering you to proceed to Calix VI immediately - is that understood?"