“I never never told him, I never told anyone”. In this moment, Damon opens himself to Elena, for the first time to anyone, about his deepest wound : his past as an Augustine vampire. I choose this moment because it finally shows us the reason that made him turn his humanity off and it also shows us that he protected his baby brother from a guilt that he didn’t want to put on his shoulders, because it was the past and he thought it didnt matter anymore… but it did matter and it changed his whole life.
Damon won and saved his life in 1958, but had to let Enzo die. He failed Enzo and had to turn his humanity off to survive the guilt. This is haunting him since that day. So when in 3x15, even tho he wins again and could let Stefan turn Abby into a vampire, take the blame and lose Elena for good, this time, for Stefan’s sake, Damon changes the rules & will turn Abby, because he’s “better at being the bad guy anyway”. What happened in 1958 made Damon who he is now, it reinforced the idea that he’s monster and doesn’t deserve happiness. And so many of his actions since 1x01 can be related to these 5 years spent next to Enzo, in those cells, and the guilt that he lives with everyday since then.