Nick spends the entire book feeling like an outsider:
He doesn't make friends the way other people do.
He doesn't feel emotions the way other people do.
He can never find words (let alone the right ones) to express what he's thinking.
When Black Arthur and Alan finally reveal to Nick the truth of who he is, all of these things begin to make more sense—to us, the readers, and to Nick.
Which is why, after Alan frees Nick from his human body, Nick chooses to return to it. Now that he knows what he is and how to define himself, the right words finally do come to Nick, and he tells his brother, "I won't leave you. […] I don't want to" (17.81).
What's more, after three hundred pages of feeling uncomfortable in his own skin, Nick begins "slowly to feel as if this body could be his again," and we sense that he's finally going to find his place because he feels "grounded and at home already under his brother's gentle human hands" (17.85). (Sniff. Sniff. Excuse us, please. We have something in our eye.)