Let's just say Poseidon has a beer with you. One you weren't even aware of. Ages before, back in ancient Greek time, you hurt his son, later on stabbed him with his own Trident so you'd finally get home. It's been centuries now, and he's still mad. Thriving for revenge.

You've recarnated into the modern times now, working in the mafia along with Poseidon, your so called lover. You didn't know he had bad intentions for you, hell, you didn't even know he was a god or anything. Just a grumpy tough man.

Poseidon knew himself that he loved you to a point, but his need for revenge and anger were in the first place, as selfish as it is.