Hi! Yes, Z and I'm glad you noticed that.
I'lltry my best to explain it in words since I wasn't expecting someone to notice it.
Before Zayn disappeared, Niall denied being in love with him since they barely had physical contact with each other. You know the saying, "you'll never know how important someone is until you lose them"- this applied to Niall and after burger grill closed, Niall became comfortable enough to refer Zayn as 'you' because of how much he missed him. He finally noticed how much he loved Zayn and by that, he dedicated every chapter as if it was for his love.
Hope you understand since I suck at explaining. Thank you for asking, don't hesitate to ask again! xx