Why would Fern want to save Wilbur's life in Charlotte's Web?

When Wilbur is born, he is the runt of the pig litter. Fern's father, Mr. Arable, wants to kill Wilbur, thinking he will probably not survive and, in the meantime, will simply use up food and energy. Eight-year-old Fern, in contrast, believes that justice demands that any creature born be given a chance at a full life. When her father relents, Fern feeds Wilbur from a bottle and helps him to survive and thrive.
When Wilbur is sold to the nearby Zuckerman farm, Fern can still go visit him. She is an especially intuitive and sensitive little girl who is capable of bridging the animal and human worlds. She can listen to and understand the conversations between the animals. She reports these to her parents, who worry she is delusion. Nevertheless, she remains a friend to Wilbur and the animals.