How is the theme of friendship and sacrifice expressed in "The Last Leaf" by O. Henry?

In O. Henry's short story "The Last Leaf," Behrman, a 60-year-old painter who lives below Sue and Johnsy, makes the ultimate sacrifice to save his friend, Johnsy. While Behrman has been a failure as a professional painter, his last work is a masterpiece. His efforts exemplify sacrifice and giving all one has to help a friend.

Johnsy, who is sick with pneumonia, has decided she will die when the last leaf falls from the building whose side she can see from her sick bed. The weather brings snow and ice, which might make the the leaves fall from the building. Johnsy's doctor tells Sue, Johnsy's roommate and friend, that Johnsy's mental state is very important in determining whether she will survive. Unbeknownst to Sue and Johnsy, Behrman goes outside in the cold weather to paint a leaf on the side of the building so Johnsy will be tricked into having the fortitude to survive. Because Johnsy continues to see the last leaf clinging to the side of the building, she decides to survive, while Behrman, who went outside in the cold weather to paint the leaf, makes the ultimate sacrifice for his friend and dies soon afterward.