How does "To Kill a Mockingbird" relate to happiness in the book?
Well, this novel is a successful slice of life. With the pain and suffering, there is always a little happiness sprinkled in if you know where to look. Before the trial of Tom Robinson, the children are very happy playing in the yard, pretending to be Boo Radley, eating Miss Maudie's cakes, and waiting for Dill to show up. They also very much enjoy Calpurnia's company since she is wise and acts as their compassionate surrogate mother. In their youth, they don't recognize any difference in color and treatment of people of different color.
Later, there is happiness to be found in everyone knowning of Tom's innocence regardless of the conviction. There is happiness in Rr. Raymond's confession to them that he isn't really a town drunk and that the bottle he always carries around with him actually has Coke in it. His explanation helps the children come to terms with the prejudice in their town. There is also happiness in the fact that Boo Radley comes to their rescue when Bob Ewell attacks. Ewell is removed permanently from the scene, and Jem, while seriously hurt, will recover from his injuries to see another day.