He was amused by the idea because his own life was sad and monotonous, and he had been treated unkindly by the world. The idea of the world being a giant rattrap—something that sets baits for people to trap them—mirrored his cynical outlook on life. He found humor and a sense of irony in how people willingly let themselves be caught in the world's traps of greed and ambition, just as rats get caught by a rattrap. This amusement also helped him cope with his difficult circumstances by seeing others' struggles as predictable and absurd.