Replying to @hopelee0 here because I don’t know how to reply to replies (that sounded weird. lol) I tried but I only replied to myself. But answering as a text post is better because I can elaborate more.
Performing Dean is the front Dean puts on to protect himself and to appear as a figure of strength and the ideal macho figure that a hunter is supposed to be. It is like a defense mechanism because he had to be Sam's parent, so it is part of what Dean believed he needed to do so that he could be that figure of strength that Sam needed growing up, but also that Dean believed he needed to be to go through everything, and it is something he continued doing through his life. Dean pretends to hate things he actually enjoys, for example, if these things could be considered weak or unmanly (like Taylor Swift music, cucumber water, or anything feminine because the ideal macho man equates feminine things with weakness). He basically tries to be like an ideal of masculinity, and represses the aspects of his personality that fall out of the hyper masculine facade.
I know this is a really concise answer, but I wanted to reply anyway. :) There are meta writers who have written extensively about this, and can explain this better, of course. It was @obsessionisaperfume who first called this aspect of Dean “performing Dean” so I believe over there you will find the better answer.
When I said that the real death of the season finale was Performing Dean, I meant it in the way that Dean opened up to his mother; I believe this definitely opened the door for Dean to start being honest about his feelings (to himself and others) and start to express himself openly without repressing. He may still have a way to go, but he has taken a huge step with that conversation.
