Screenshot from the game: BioShock Infinite + Adobe Photoshop.
She looks so changed here, now that her power to see the whole multitude of worlds is released. She looks far less cheerful and more tired than before.
I believe it's the burden that she carries. She knows what happens when Booker chooses to not go through with the baptism, and she knows what must be done. Although it is still the saddest ending I've ever experienced, it's also the most powerful, and the most perfect. The ending was exactly what it needed to be.