After Joel told Ellie he would've still done it all over again; I think that was the first time Ellie fully realized that his love for her was eternal. She always knew he loved her, of course. But I think that up to this moment, she just wasn't fully aware of how much.
This look on her face here is a look of harsh realization. Peering into his eyes at this moment, Ellie saw nothing but love.
Because even after all those years of losing her due to his actions, he'd still choose to go through with it all over again just so she could have a life; even if it meant for him not to have her in his. That is pure love.
Nothing could've ever wrecked me more than Joel saying, "I'd like that" on the verge of crying after Ellie told him she would like to try to forgive him. The way he looked at her in this scene, you can only see that her forgiveness meant everything to him. And that just fucking breaks my heart knowing he didn't live to witness it.
