Explanation:
The character in the story "The Rattrap" by Selma Lagerlof is a peddler who has a somewhat cynical and pessimistic outlook on life. He has been traveling around selling small trinkets and is often met with rejection and indifference from people. As a result, he has developed a negative view of human nature and believes that people are inherently selfish and only out for their own gain.

One day, the peddler comes across an old ironworks where he seeks shelter for the night. The owner of the ironworks is kind enough to let him stay, but the peddler is wary and suspicious. He believes that the owner will eventually demand payment for his stay and sees it as a trap.

It is in this context that the peddler comes up with the idea of the rattrap. He compares the ironworks owner's supposed trap to a rattrap, where unsuspecting rats are lured in by the promise of food and then trapped. The peddler finds this comparison amusing because he sees himself as the rat who has fallen into the trap. He believes that just like the rat, he has been fooled by the owner's kindness and will eventually be trapped and punished.

The peddler's amusement at his idea of a rattrap stems from his belief that it reflects the exact situation he finds himself in. He sees himself as a rat in a larger metaphorical trap, where he is constantly being deceived and taken advantage of by others. The comparison is humorous to him because it confirms his negative view of human nature and reinforces his belief in his own unfortunate circumstances.

In summary, the peddler's amusement at his idea of a rattrap is based on the belief that it accurately represents his own situation of being trapped and deceived by others. It is a reflection of his cynical outlook on life and his negative view of human nature.