Nick's entire life is focused on Caitlin, likely because of the abuse and unhappiness he suffers at home. With Caitlin, he finds peace and happiness; however, he isn't willing to direct those positive emotions back at her. When they break up, he goes through a series of emotions that echo withdrawal. He feels discontent, restless, and unhappy without her.

One example that shows how Caitlin is like a drug for Nick is how his need for her grew over time. At first, he was content with occasional and normal levels of interactions for high school students. Later, though, that wasn't enough to satisfy him. He wanted more time with her, even when it interfered with other areas of their lives. Caitlin, on the other hand, wanted a normal life and had other goals outside their relationship. Nick saw these things as getting in the way of their time together.

Nick hits Caitlin because he imagines that she believes he isn't her true love. He doesn't want her to focus on anyone else but him, because he is completely focused on her. His love becomes abuse. His concern becomes violent, overbearing, and accusatory. Like a drug addict, he's always scared that something will separate him from his drug.