Choice is the biggest part of it. When Mordred mentions she has a choice, Katniss pauses and reconsiders. She isn't forced to go to this festival. She isn't going to be put on display and meant to pretend that life in the Districts is Just Fine. She's just going as herself for as long or as short as she wants.

"Well, I guess I could go. I did put some work into it and like you said, no one's making me. I can leave whenever I want. As long as I have the choice and I'm not being forced, it might be okay."