In chapter 12, Lorraine notices that John is more concerned with his appearance and thinks that it might be because he has feelings for her.
Until John kisses Lorraine while they play at pretending to be adults, she thinks that she's only his friend. She thinks there's nothing else between them even though she has feelings for him. This platonic relationship was easy to maintain and when it changes, things change between them. Paul Zindel writes:
But suddenly we had become slightly awkward in front of each other. Of course I had always been clumsy around him, but at least I knew I had been in love with him for months. I also knew he liked me a lot but only as a friend or a dreamboat with a leak in it. But now suddenly he was wearing shaving lotion, combing his hair, and fighting with me. There was something about all that which made me smile as I scraped the Sloppy-Joe off the plate.
John's interest in his appearance is a signal that he has feelings for her. He wants to impress her by looking good, smelling good, and generally being appealing. When he wasn't interested in her, their relationship was less conflicted but he also wasn't as concerned with his physical appearance. The combined interest in that—along with the fighting—lets her know that things have changed.