What is the moral that we get from this story?

I might consider the theme of 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.