On such a beautiful morning, why did life seem like a burden to Tom?

Life seemed like a burden because Tom was beginning his punishment. He must whitewash a fence after being caught sneaking into his bedroom window late at night. Aunt Polly wasn't happy.

He got home pretty late that night, and when he climbed cautiously in at the window, he uncovered an ambuscade, in the person of his aunt; and when she saw the state his clothes were in her resolution to turn his Saturday holiday into captivity at hard labor became adamantine in its firmness.

Tom appeared on the sidewalk with a bucket of whitewash and a long-handled brush. He surveyed the fence, and all gladness left him and a deep melancholy settled down upon his spirit. Thirty yards of board fence nine feet high. Life to him seemed hollow, and existence but a burden. Sighing, he dipped his brush and passed it along the topmost plank; repeated the operation; did it again; compared the insignificant whitewashed streak with the far-reaching continent of unwhitewashed fence, and sat down on a tree-box discouraged.