Why

She actually liked him but Grayson said that you need to distinguish someone you marry and have sex with. So she was hurt and hated Grayson ever since.

Kiss me liar explain there dynamic a little more. grayson basically told her he will never take her seriously. Cos, she's a beta. In the most douchebag way possible.

He did hurt her ego, that's why she has a grudge against him.

Naomi genuinely considered herself a true friend to Grayson. She wasn't in love with him, but she had a massive ego. She thought so highly of herself—believing she was wonderful enough for sought-after alphas like Keith and Grayson to keep her around as a casual hookup—that she decided to test Grayson (purely to feed her own ego) by asking if she could seduce Chase. She expected to have some fun making Grayson jealous and to feel a sense of pride in her powers of seduction, but his response caught her off guard. Grayson told her there was a difference between people you have fun with and people you marry...

To continue... Grayson hadn't meant to slight her with that answer; he was simply being consistent with his own beliefs. First, he knew Chase would never get involved with someone just for fun—not even to extract pheromones. Second, Grayson had always believed that an omega was destined for a specific alpha, and that such a union was meant for marriage. This view had been shaped by watching and romanticizing his own parents' relationship—an alpha-omega pairing. Regardless, Naomi took his response as an insult; her ego was deeply wounded, and she vowed to exact revenge. From then on, she detested him, yet she pretended to be his friend so she could watch his downfall up close when he was eventually rejected.

Oh it was her wasn't she also with keith ?