What is Behrman's masterpiece? What makes Sue say so?

Behrman, a 60 year old painter, had dreamt all his life about painting a masterpiece. However, it was not until Johnsy fell ill and Sue discussed it with Behrman that he found the opportunity of accomplishing his dream. When Sue discussed her hopelessness in convincing Johnsy, Behrman decided to paint the leaf of the creeper with the fall of which Johnsy had associated her death. It was evident that the leaf would fall in the violent storm. Behrman painted such a genuine leaf that Johnsy was amazed to see the leaf intact on the creeper even in adverse weather conditions. It made her realize that it is about one’s own determination and that her health would get better if she wished for it.

However, it is tragic that the man died of pneumonia as he had painted the leaf while in the storm the entire night. Sue called it a masterpiece because it saved Johnsy’s life and convinced her that things are not always as bad as they seem to be.