Why does Johnsy count the leaves?

She counts the leaves to keep herself entertained in

bed, since she can't paint while she is sick.

She counts the leaves to annoy Sue, who is doting on

her too much and won't let her rest.

She compares herself to the falling leaves: she believes

that when she counts none left, she will die.

She counts the leaves to measure when winter will

arrive; when it does, she will die.