And his little fling with Jaimie was pretty much the last straw. If Dean's going to potentially fuck up their team by fucking another member, Carter wants it to be him. Wants it bad enough that it scares him a little.
Because he knows himself. Knows that he wants to own Dean in ways he's never felt before. He didn't feel this need with Nicole. The possessive side of his nature was satisfied every time he looked at her hand and saw his ring there. That isn't an option with Dean for a lot of reasons, so he wants to leave his mark on him in other ways. He's never seen Dean visibly marked up, but he wants to mark up Dean so that everyone can see he's taken.
Carter knows he has a hard time letting go. It was why he held off on giving Nicole the divorce she wanted even after Steven, it's why he still talks to Nicole and Anna, hell, it's why he still sees Anna's kids when he gets the chance. And if he ever gets Dean into his bed, he's never letting go.