Life is hard, but it's better with company. Amy and Matthew are unlikely sidekicks, given that each of them has a different inhibiting condition, Amy with her cerebral palsy, and Matthew with his OCD. Although their limitations often leave them lonely and misunderstood, they find solace in each other's suffering—not because they want the other to suffer, but because when they talk about how difficult life's challenges are, there's an analog. They understand each other. For them, this realization is so powerful that it quickly blooms into a lifelong desire to be together, in love.
This is the core idea of the novel, that although life can be cruel and unfair, it's really difficult to regret life when you're in love, when you're being supported by people who understand (more or less) how hard your time has been. This kind of community is often at the center of romantic love, and when it's not there, the couple knows it. So in one regard, their plights did have a plus side—the difficulty of their lives made it impossible for them to take each other for granted, and the intimacy that develops is truly remarkable.
This is a testament to the resilience of humans, that even when life is difficult, we persevere by loving and supporting one another. When we have a community, we become invincible. This is an important reminder for people who feel alone, because as these seemingly unlikely friends could tell, no one is really alone if they find the right friends.
