In The Last Leaf by O. Henry, what makes Behrman's work a masterpiece?

Behrman's painting of a leaf on a vine that was being destroyed by a heavy storm becomes a masterpiece, not only because of its accurate and realistic depiction of a leaf on a vine, but due to the context in which the leaf was painted and the outcome the painting achieved.

Behrman did not paint the leaf because he wanted to achieve fame and fortune, although it had always been his desire to complete such a painting, but because he wanted to save a life. Furthermore, he had sacrificed his own to achieve this goal. The story makes it clear that Behrman performed this selfless deed out of love for his fellow man.

In the story we quickly learn that many artists of all kinds had come to Greenwich Village to practice and ply their trade, but that the community had been ravaged by  pneumonic plague. One of the artists so infected was Behrman's upstairs neighbour, Joanna, or 'Johnsy', for short. Johnsy's live-in companion, Sue, was told by the visiting doctor that she would die if she did not have hope. She stood at least a one-in-five chance of survival if she had some purpose to cling to.

Sue discovered that Johnsy had been counting the leaves on the wall of the building opposing theirs. She told Sue that if the last leaf went, she would go too. Sue imparted this information to old Behrman who was quite aghast that Johnsy would allow a leaf to determine her fate.

When Johnsy looked out of her window the next morning, she noticed that one last leaf was still clinging to the vine and when she did the same later, she was surprised that it was still there. This inspired her and she decided to fight her illness. When the doctor came he announced that she had a fifty-fifty chance of survival and needed some care.

He also informed the two girls that Behrman had contracted acute pneumonia and that he was old and frail and would have to go to hospital. The next day he told Johnsy that she was out of danger, all she needed was a food diet and care to make a full recovery.

Susie later informed Johnsy that Behrman had passed away after only two days. She told her that they discovered that Mr Behrman had gone out into the terrible weather with a ladder, paintbrushes and paint to create Johnsy's last leaf, and so contracted the deadly illness. As Susie put it:

 "Mr. Behrman died of pneumonia to-day in the hospital. He was ill only two days. The janitor found him the morning of the first day in his room downstairs helpless with pain. His shoes and clothing were wet through and icy cold. They couldn't imagine where he had been on such a dreadful night. And then they found a lantern, still lighted, and a ladder that had been dragged from its place, and some scattered brushes, and a palette with green and yellow colours mixed on it, and - look out the window, dear, at the last ivy leaf on the wall. Didn't you wonder why it never fluttered or moved when the wind blew? Ah, darling, it's Behrman's masterpiece - he painted it there the night that the last leaf fell."