[Upset Katara is upset. A statement like that rips the rug right out from underneath her, and she really wants to yell at him and call him on it. But he's so obviously upset that she is going to try to be calm.]

...you don't know that, Sokka. How could anything take our memories and all our feelings and just -- change them? It's not as if we'd fallen in love with some strangers. It's Zuko and Toph.

[...and ohcrap, did she just admit that she was in love? To Sokka? ...time to move on quickly.] You should go after her.