“But I haven’t changed.” - “And you never will, River.” - This is an important line because of how he says it. River is young here - so very young, and the last time he saw her, from her viewpoint was very up and down. He tried to push her to restart time, he lied and was seemingly angry with everything she’d done. And yet, here he is not only telling her that she won’t ever change - he’s telling her by how he says that that he is more than okay with that. He likes who she is. All of it. Every bit - and this is technically a spoiler. But he tells her anyway, because it sort of cements that she never will change, doesn’t it? The Doctor told her - she will always be like this, so she is. And he tells her so she will. And that’s huge for him, because you have to think - all of the things she is - flirty and smug and mad and clever - are all things that drove him mad in the beginning. But he doesn’t care, because it’s everything he adores about her now.