The Last Of Us Explained: What Does The Ending Really Mean?
1. Loss And Sacrifice Are Integral Elements Of Survival
In order to live on, you have to learn how to let go. Joel never learns that - he still keeps the broken watch that he got from his daughter on his birthday 20 years ago. He also projects his feelings towards her on Ellie and refuses to let her go as well.
Emotional attachment lowers one's chances of survival. Bill is a loner, who sacrifices his relationship with his partner in order to stay alive. Ellie calls him paranoid for setting so much traps, but in the end, it's Bill who survives, while his partner is dead.
Marlene knew Ellie's mother, she even promised her that she'll keep the girl alive. But in the end, she knows how to let go. If it wasn't for Joel, she would probably save humanity.
That's why in order to survive one needs to forget everything about his former life and just focus on the present, as without it there's no future. Whether it's moral to do so is up for debate, but there's no doubt that forgetting and letting go gives one a better chance of surviving.