Treating people equally is seriously difficult
The natural assumption is that if you can be friends with a mentally handicapped person or a "cool," "normal" person, most people would prefer the second person. But that is prejudiced behavior, akin to racism. That's not to say that living in community with the differently abled is necessarily easy—for instance, David has screaming fits that embarrass Catherine—but it's still an important part of learning to love people well.
Life with an autistic family member is a unique challenge, but a meaningful one
Autism is a difficult aspect of life. Austism within the family usually means ostracism from normal society, and just having to go to David's OT with him means that Catherine has less time for things like friends and basketball. But ultimately, the process of learning to love David means that Catherine is better at appreciating people than most people are, and so she learns to give back.
It's easy to use people for our own emotional needs
Friendship is a strange process, and sometimes, we can come to relationships like Catherine does with Kristi, with a specific need in mind that that other person might be able to fill, but ultimately, that just leaves her disappointed, and Kristi is also less successful than she might have been because Catherine wants Kristi's approval, and Kristi is just as confused as Catherine, so she makes mistakes when confronted with Jason's handicap.
Being there for people who don't have the same privileges or abilities is a big part of moral goodness
Ultimately, this is the main theme of the story. Catherine is a hero because she (literally) gives a voice to a voiceless minority, those not as enabled as the rest, either through mental or physical handicaps. She finds the most meaning when she prizes those people instead of being shy about them.
