The Last of Us is a game about choices, yet the player largely has no agency in those choices. This is important as it allows Joel to be created as a character that is Joel and not a hybrid of himself and the player, as happens in most games that allow player choice in certain, but not all, instances. The player is not Joel. You are playing the game to see what Joel does for himself.
The precedent for this is set for this in the prologue where Joel forces Tommy to drive past a group of people with a child who are trying to hail a car. All the player can do is watch and question whether Joel was right and if they would have acted differently in that situation. This moment perfectly summarises Joel’s character. He is not trying to save the world, he is trying to save himself and those that he loves.
Joel’s morals and the player’s lack of control ultimately comes to a head in the conclusion to the game. Joel decides that he cannot allow Ellie to die, even if it means eliminating the one hope there appears to be for some kind of cure. The player is forced into this in a confrontation with a group of doctors. The only option here is to kill them. Joel has decided he has to save Ellie by any means and you are forced to enact these means. The only way out of that is to look away, to stop playing the game and leave Joel to his own choices.
This whole sequence forces the player to judge Joel. Why would he do this? Is he being selfish? Is what he is doing right? None of these questions are directly answered by the game. Instead answers are implied by Joel’s character that has formed throughout the game. If the player had been allowed any say in the key choices of the game there would then be a sense of ownership over Joel. He would not stand on his own as a character with his own motivations and reasons for his actions. The previous questions would not then be addressed at Joel but at the player. By not allowing the player to influence his decisions and his morality Joel is then an individual. He is not a mix of player and character like Lee from ‘The Walking Dead’. He is just Joel. His reasons for what he did are his own. They can be speculated on, but only Joel can truly know why.