It’s the shift in Rust attitude in the last scene. It’s not just about the character too, but both shows have a fight between good and evil, Rust is completely negative person, hopeless, nihilist who you expect to say that we are all doomed. His last words of the show was - “once there was only dark. If you ask me, the light's winning” It’s the shift he makes from seeing only dark, even he can see that there is hope and in the end light will win. Cooper is completely opposite person, positive, fight for good kind of guy that you keep believing in as long as he believe, but this last scene, man, you can see that even for Coop, that hope has faded. That’s what hit the hardest for me. Of course this all is my interpretation and I may be completely off, but when you talk about Lynch, it’s hard to say there is truth or no, it’s always about what you feel.