The reader who will fall hardest for What If It's Us is someone who loves the idea that a single, serendipitous post-office encounter between two very different boys—one clinging to Broadway-style cosmic certainty, the other certain the universe has terrible timing—might spin an entire summer into a rom-com full of missed connections, awkward do-overs, and the delicious suspense of whether destiny actually knows how to write a satisfying final act.