I'm honestly surprised jimmy is still holding strong to those beliefs. Jag's pact with him seems pretty one-sided in favor of jimmy, so either he's got enough ego to counteract that self-loathing when it comes to what he provides to the relationship, or he's so stubbornly self-righteous that he can't fathom any other explanation, even when the evidence doesn't add up.
honestly I'm just surprised jagged still likes him. Love really is blind.

Jimmy figures Jag's reasons for staying with him, is the same Monkey's reasons are for being drawn to Jag: safety & personal benefits.

Jag isn't a complete idiot, he knows they wouldn't let a demon like him run around freely. So he has a choice of working with the grumpy agent and enjoying at least some privileges out of that, or ending up on the Agency's hit list and having to fight or hide to stay alive. He feels the hostility humans have for his kind, (reasonably so, for the most part. I mean they do eat & possess people.) And he doesn't exactly have any other allies, being the amnesiac he is. At least Jimmy seems to care enough about him to keep him alive. And doesn't keep him on too short a leash, either. Agents like Dimitri and Gerard sure as hell wouldn't let Jag roam the city at night, he'd get literally chained up. Or just sent straight to the lab to be experimented on for the rest of his existence. Jag has no friends in the Agency, except for Jimmy.

All of that is Jimmy's reasoning, that whatever Jag does is at its core an act, to fool him and/or the Agency into trusting him. (Though tbf Jimmy figures that for most humans too, the guy has major trust issues. :U )