Because when he had his first rut, Grayson paniced because there was no one there for Chase to spend it with. So Grayson got is dog Alex ( which is a crazy thing to do), but nothing happened and Chase passed out. Chase remembers next to nothing, but remembers that a dog was present. So he is uncertain if he slept with a dog or not, and no one told him him the truth. And Grayson uses that to make Chase go to the pheromone parties, where they take out his pheromones with an injection.
Spoiler.....
Chase does feel like he spent his rut which is why he is more scared and he did spend his rut with someone and that person was actually josh...
Anyone spoil me why he's scared of dogs?