In the January 26 journal entry in Breathing Underwater, Nick refers to himself as an “addict.” What is Nick addicted to?

Nick is addicted to Caitlyn. He describes her as both his drug of choice and his dealer. When they were together, they just couldn't get enough of each other. Each day he'd pick up her up and drop her off at school, and then later drive her home. He'd call her every day, whether or not he had football practice. She was taking over his whole life.
It seems that Caitlyn was equally infatuated with Nick. (That is, before he started hitting her). She took the locker by his at school, would sit on his friends' regular bench at Mr. Pizza, and would gladly make out with Nick in his car. All in all, it was a pretty hot and heavy relationship, driven on by a deep mutual passion. But then Nick went and blew it all by hitting Caitlyn.