Her gray, sun-strained eyes stared straight ahead, but she had deliberately shifted our relations, and for a moment I thought I loved her
This can be connected to many high school dramas. People get very easily confused about love and relationships. This is an important part of Nick's life because this is a rare moment when his life is seen, and the main focus is Nick instead of being the narrator. His tangle with the other girl causes confusion and he needs to untangle his problems.