This is a winning gesture of affection, but also of respect and admiration, and it defines the romance that drives the movie. You can see why Ally is moved, and you buy it. Jack believes in her. He asks her to join him onstage, and it's crucial that he doesn't push, or insist, and trap her. You can come onstage or not, he says, but I'm going to sing it anyway. And when she does join him onstage, he cedes it to her.