Why does Marty sleep on the coach in Shiloh?

In "Shiloh" by Phyllis Reynolds Naylor, Marty sleeps on the couch because he feels a sense of guilt and conflict over his family's situation and the dog, Shiloh. He struggles with the decision to keep Shiloh a secret from his parents, particularly his father, who has a strict view on ownership and responsibility. Sleeping on the couch symbolizes his emotional turmoil and the burden of his conscience as he navigates his love for the dog and his loyalty to his family.