They really are more similar than they realize. Elsa finds herself empathizing too easily; having to hide yourself away (in Elsa's case more literally, perhaps, but it's all the same in the end) hits close to home. Trying to reconcile after years of doing that can be hard. Even though Anna fully forgave her, sometimes it still felt insurmountable, the issues they had to sort out. There should have been a relationship between them, grown naturally, but they'd had to start from scratch instead.

"It would be easier, but I don't think you'd be happy that way." Which she knows is easy to say, but not easy to do. "I don't know your brother, but if he cares about you, he'll want to learn more about you now, while he can. Even if that's difficult for you both."