Shawn, the protagonist of this story, is an adolescent boy with cerberal palsy. This means that Shawn has little control over his own body, which is twisted and hard to manipulate because of the disease. Shawn also gets seizures as a result of his disease. People at large tend to view him as hopelessly disabled and retarded (his speech has been affected, too). Shawn's own father, unable to see the condition of his son's body, abandons the family early in Shawn's life.
Even considering all this, Shawn loves life. As a teenager, he is trying to find his place in the world - his individuality, complete separate from his disease. He enjoys TV, music, friends, games. He likes experience. He gets angry, has happiness, forms bonds, and even develops a crush on a girl (his sister's friend). In other words, Shawn is a normal 14 year old. Why give up on life when there is still pizza in the world? He simply wants to get the most out of life that he can, despite his disability. His father, considering a mercy killing, can't understand that even though a person has problems, it doesn't mean that that person isn't happy.