When Nick says the quoted line, he realizes how unintentionally therapeutic writing the poem was for him. The fact that he wrote things that he didn’t realize he was thinking and feeling shows that writing allowed him to process his complicated thoughts and emotions. Earlier, he noted that his hands that were typing were “apparently incommunicado” with his brain, which also suggests that in writing, he was able to sort out how he was feeling without filtering his true thoughts.

The poem he wrote shows how alone and overwhelmed he felt in his life and how stifled he felt in his relationship with Tom. It shows how hard he worked to present a tough exterior to his friend, to act like he was ok when he was not:

“And I stand alone,” he writes, “drenched in sweat and untold secrets; But slapping your hand, saying everything's fine.”

After he writes this line, Nick reflects on the history of his relationship with Tom, and questions what might have happened if he had told Tom what he was going through at home. He shakes off this thought, assuming that Tom would have laughed at him. But in the end, Tom confronts Nick about the poem and says:

You hurt Caitlin. You hurt her bad. That's all I saw. I didn't know you were hurting too. I told you everything, and you kept this huge secret from me.

Tom’s sentiment here shows how the poem taught him about Nick’s true feelings that built up and led to his violent behavior. The poem reveals that Nick felt isolated and vulnerable, even though he kept these feelings hidden from Tom.