
*** SPOILERS AHEAD****
Basically grayson lied to chase about him having sex with a dog. The reason why he lied isnt too clear in the translation i read off. But thankfully its not true and never happened. The incident was at the time of the rut which we see at the start of the manhwa, the “dog” he slept with was actually Josh but Chase had no memory of the night and Josh ran away and Grayson thought it was a good idea to go “you fucked a dog” (I HAVE NO IDEA WHY I PROBABLY MISSED IT LOL)

Grayson really lied but during that time it was Chase's early rut when he was young, he was supposed to sleep with someone (forgot the name but it's not josh) but he fell asleep and forgot what happen. Now, Grayson lied which traumatized Josh, the reason? Nothing, he thought it'll be fun which will make Josh mad at him... About the night Chase had sex with someone, Chase is aware that he had sex with someone (Josh) that night, that's why he's looking for him.

No reason, only because Grayson's a psychopath. He doesn't feel emotions. Everything he did for Chase, he thought he was 'helping'. His way of helping is psychopathic ╮( ̄▽ ̄)╭ Their dad has always taught them to release their pheromones for fear of the consequences. For years, Chase did fuck all and only did the shots which is very dangerous. Grayson's idea of helping him are forcing him to go to the parties and even then he doesn't release pheromones. Grayson thought it was a good idea to scare him like that

Spoiler, the anwser above I now read a few times (which I find strange), but it's not what really happened.
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It was Chase's very first rut, not the time he slept with Josh (he knows he slept with an omega then, but that's all).
When Chase had his first rut, there was no one around to spend it with, and Grayson paniced and brought his dog Alex ( as a last resort, in his mind), but Chase passed out and nothing happened. They took out his pheromones by injection.
Chase remembered next to nothing, only that a dog was present. And no one told him the truth.
And he believed what Grayson told him, because he remembered a dog.
Why did Grayson told him something like that? He uses it to force him to go to the pheromones parties, even if he doesn't have sex with anyone there, it's still the best option to release pheromones without having sex, without going to the hospital, because medical stuff is present (often Steward himself) for emergencies, who can remove the pheromones by injection. Chase always passes out at those parties, and then they use an injection to remove his pheromones.
Chase is not willing to have sex with anyone and he doesn't remove his pheromones any other way. And normaly dominant aphas can controle if their sperm is fertile or not, but Chase has problems with that (-> Josh), too.
But pheromones accumulation is very dangerous for dominant alpha, from memory loss- braindamage-going totally crazy/dieing.
And in the Miller familily the father already suffered brain damange, in his case the ability to feel emotions and becoming dependent on his omega partner. Releasing pheromones is important in this family.
Grayson was born manifested, so as a kid had problems with right and wrong, is unable to understand and recognize emotions etc. So his way of helping can be helpful/weird/dangerous/the opposite of helping, and he is not even aware of it.
And that resulted in Chase's dog trauma.
Can someone who read the novel spoil this whole “dog thing” it was in kiss me liar too and I’m like super freaked out. What type of beastilatiy situation is going on here