The most important lesson that Jess learns is that he is not alone.  As a middle child and the only boy, Jess has always been isolated from his family.  The same is true at school - the other kids don't connect with him.  The fact that he is an artist doesn't help him in this situation, for it is considered a "girly" activity.  Jess is determined to be the fast runner at school because he is looking to be accepted and to belong.

Leslie teaches Jess that he can be exactly who he is and not be alone.  Leslie reaches out to Jess, and he responds to her, and this is the start of Jess's learning curve.  The more he interacts with Leslie, the more he sees that he just needs to reach out to those around him - whether it be May Belle, Janice Avery, or his father - and he will be accepted and appreciated.  Jess's trip to the museum with his music teacher is the climax of his lesson.  He has allowed Miss Edmunds to see him as an individual, and it leads her to invite him to the National Gallery.  Jess is fully accepted and is able to explore an interest that fascinates him.

In a sad turn of events, it is Leslie's death that reinforces this lesson.  After her death, Jess finally connects with his father, who provides comfort for his son.  Jess is able to recognize how important Leslie's lessons were:

"It was up to him to pay back to the world in beauty and caring what Leslie had loaned him in vision and strength."