But that whole scene in the cave, the idea of "let the quest end here" and "happy endings can't come in the middle of the story" and the idea of "there are no happy endings because nothing ever ends" continues to stick with me to this day. Even the ending itself, yeah it seemed like a happy ending on the surface, but my little-kid brain started to think more and more about it. Was it really a happy ending? I think that was the moment I started to think deeper about endings in stories, especially the "happily ever after" cop out, because this was a fairy tale that didn't end that way.
I remember you saying this about the Last Unicorn some time ago. I recently watched it for the first time in years and I agree with you that it is pretty profound.
The most profound to me as a child was definitely the death of Little Foot's mother in The Land Before Time. Though I didn't understand what was going on at the time, I'm astounded to this day on how much I find I empathize with Little Foot's pain, and his struggle to continue despite his tremendous loss. Such a beautiful movie.
As an adult, I think I was probably most affected by Life Is Beautiful, and all the effort the father (whose name I forget) puts into keeping his son happy and positive despite the horrible things that are happening around them.