My sympathies. I know the effect you mean. I have probably made it through the worst experience in my life, so a lot of things below that threshold just don't touch me in the same way.
I'm glad you had that much time with him and that his mind was unclouded. Even now, years later, I treasure those moments of presence.
Strangely enough it left me immune to images and videos of human suffering, and they now have no effect on me at all.
But I can't deal with anything to do with animals in pain or suffering.
I wonder if it's something to do with communication. He would communicate what was happening very clearly and was very rational about his wishes.