As far as she knows, she isn't sure it'll be okay. But that's part of the reason she puts so much trust in Joy, isn't it? Joy knows what optimistic things to say to help her feel better. Her confident words cause Sadness to smile a bit, and she says in return, "Yeah... I know." She doesn't know for sure, but she knows that things will be as good as they can possibly be, so long as they work together like they have been.

Tomorrow is another day.

But they have each other. They'll be okay... right?