In order to provide some guidance for your question, let us begin with a look at how The Oxford Dictionary defines the verb rationalize: “To attempt to explain or justify one’s own or another’s behavior or attitude with logical, plausible, reasons, even if these are not true or appropriate.”

We come across an instance of rationalization in the first chapter of I Am a Cat. The cat who narrates the story tells the reader how he was taken out of the warm basket he shared with his siblings and “dear mother” by a shosei (a student who does minor chores in exchange for room and lodging) and “pitched with violence into a prickly clump of bamboo grass.” As a nearly helpless kitten, the narrator is stunned, confused, and very hungry.

Driven by intense hunger, he manages to enter the kitchen of a house where he meets an unkind servant woman named O-San. “No sooner had she seen me than she grabbed me by the scruff of the neck and flung me out of the house.” Hunger and cold drives the kitten to return to the kitchen. O-San flings him out again. The process repeats itself several times, until the master of the house tells the servant woman that the kitten may stay.

Understandably, the cat bears a grudge against O-San for her cruelty. He explains how he dealt with this feeling. “The other day I managed at long last to rid myself of my sense of grievance, for I squared accounts by stealing her dinner of mackerel-pie.” Rationalization can be a kind of skewed and self-administered justice that, at least in the mind of the aggrieved person, sets things right.

Unfortunately, people are not always able to clear up differences or have conversations about things that have gone wrong between them. When one person feels like he or she is owed something, rationalization may come into play in order to settle the feeling of a grievance within oneself. The emotion-influenced reasoning involved in rationalization can give a person’s conscience permission to do something he or she might otherwise consider to be unethical. This is clearly a human trait that is cleverly portrayed in the anthropomorphized narrator of the cat.

How does the cat in I Am a Cat rationalize stealing the fish?

On page 24, the cat says he gets back at a "servant-woman" who keeps throwing him out of the house by sneaking in one more time and eating her dinner of mackerel-pike. Like a human, he rationalizes the theft by stating that, first, he was cold and hungry, and so therefore it was his animal right to help himself to the food, and secondly, he claims that the woman deserved it. If she had been kinder to him, he wouldn't have felt the need to seek his revenge and eat her dinner. It is reminiscent of when humans commit crimes on the basis that they (the criminal) are the victim and that the person they are stealing from is at least partly responsible for their bad luck.