Deok-Im calculate her gains and loss. It's pretty clear that she lost more than she gained, that's why she often asked him if he loves her. Because his love is still and is the greatest gain. Yet she never said back the word to him, cos if she do it's another loss.

The box where all her belongings are, is where she kept her feelings for San hidden. Away from him so he wouldn't know. To protect her remaining pride. To say at least that she did not lose all of herself to him. That she preserved a part of herself for her only.