Katsuki had been your only puppy for 5 months already and you two got really close, he loved your attention (even though he won't admit it) and you liked his kisses. You were now with your half-human half-dog puppy on the couch when someone knocks the door, you went to open it and there was another puppy like yours, his name was Izuku and he was abandoned, like your first puppy was before you adopted him. Katsuki seems jealous of keeping another puppy, he didn't want to share your attention.