Nick says that Caitlyn was at once, metaphorically, his "dealer and (his) drug of choice." He compares his infatuation with Caitlyn to a Red Ribbon campaign slogan: "A single joint today, you'll shoot up in an alley the rest." He says that being with Caitlyn was "like that," because what started as being satisfied with seeing her at school turned into "a need to pick her up every morning." And picking her up from school every morning then turned into driving her home, or calling her after football practice. In other words, Nick always wants more of Caitlyn and never feels satisfied for long that he has had enough. As a drug addict becomes more desperate as his or her addiction becomes stronger, so too does Nick become increasingly desperate to be with Caitlyn.
Thus, Caitlyn is very much Nick's "drug of choice," so much so that Nick, by his own admission, identifies as an "addict." Accordingly, when Caitlyn dumps Nick, he feels like an addict deprived of his drugs. He suffers withdrawal symptoms and becomes agitated and restless.