Due to historical factors, Canine folk and Feline folk generally do not get along. Sure, there's people that overcome their differences and become friends. But the two demographics are generally somewhat hostile/distrusting of one another.

If I had to say it bluntly, Luka is mildly prejudiced towards dog-folk. It's an old fashioned mentality from his homeland that Natasha does her best to correct whenever she sees it. The severity of his behavior depends on whether or not he's seen dogfolk ragebait online that week.