Background:
Danse is my favorite companion and the only one I've romanced. I've travelled with him for close to 300 hours now. He's not perfect and some things about him bug me, but he can be endearing and I like him as a character.
Story:
A couple of nights ago I dismissed Danse in Sanctuary and left him there so I could go try out the Institute ending.
Hours of gameplay pass and I've made it to the last bit of the Institute MQ when they send me to the airport to kill all the BOS there and take down the Prydwen. At some point during the quest, toward the end of it, I'm standing on a high vantage point using VATS to find/kill as many BOS as possible. They're down below and it's dark so I can't see many of them without VATS.
So I'm efficiently killing them when I VATS onto "Paladin Danse." I use my scope and sure enough, it's him. He's attacking the synths and they are tearing into him. I can't keep watching, though, because I have other things I have to do for the quest, so I leave him knowing this has to be the end for him.
As soon as I can break away I run down to Danse who is slumped on the ground by this point and I try to stimpak him, try over and over to talk to him, but he has no dialogue for me and only about 10% of his HP left.
However, I can't stay with him, I can't comfort him, I can't apologize to him, I can't do anything. I have to get back to the quest so I don't fail it and have to do this all over again. I leave him there on the ground, knowing he's going to die. It's killing me.
The synths and I complete our mission and my first thought is Danse, but as I turn to run down to see what happened to him, so I can "say" goodbye and get some closure, one of the Coursers tells me it's time to leave and immediately relays me to the vantage point where I watch the Prydwen crash into the airport in a giant fireball right onto Danse
I couldn't help it. I cried.
So, I won't be doing that ending again.