I mostly feel confused by Cang Yu's inability to sit still. I suspect his boredom in the story may reflect the author's boredom with his own story near the end. Dude has barely lived a normal modern mortal's lifespan and he sounds ready to commit double seppuku.

The biggest part of the problem isn't the physical age gap or even the mental age gap but the bridging the mentality of two different species. ML is already in his 20s (or 420s) when he falls for CY. The problem is that he spent the majority of those years in ignorance as a normal animal and the rest as a very cold intelligent species that doesn't care to understand too much of human nature. The saving grace here is that there is absolutely nothing like grooming going on despite large physical and mental age gaps (latter reversed in last half of story). CY took a very hands off approach in raising the cub and as a straight man never considered the child as a potential partner. His initial feelings stemmed from an unfair emotional dependence on the 'child'. ML is ultimately the one person CY feels comfortable being himself around. This feeling obviously fades over time but in the story gets replaced by other emotions. Sketchy aspect is that the ML, despite aging up and maturing beyond CY, is still the child in the relationship thanks to his unconditional trust and love for his guardian even if these emotions stemmed from moments that had nothing to do with their past. The relationship still has CY take advantage and unscrupulously rely on the ML's naivety no matter how many times the author clarifies that CY never saw ML as a real child to justify that CY did not fall in love with a young boy just because he was young or a boy.