I think Bella's change was like driving the message home to Jasper. He had watched his brother resist the most tempting blood in his existence. He may have tried to write it off as simply the fact that Edward has had decades of experience controlling his bloodlust. But I don't think for a second that he believed that 100 percent. I think Jasper had a seed of doubt planted in his mind when he realized just how much a vampire can control himself if he really wants to, as he watched Edward do it.
Then with Bella reacting so easily to the change, Jasper had to admit to himself that there is just simply a level of control that he is not enforcing on himself. Sure, Bella had preparation. But Jasper's also been trying to adjust to the vegetarian lifestyle for a while now. He should, by this point, have mustered up enough control in himself to resist the every day urges better than he does. And now he's faced with that truth. Now he has to expect more of himself as a full-grown vampire. It's good for him. He needed these bits of proof to push himself so that he can expect more of his own abilities. He's a great character, but his background has him marred with doubt and warring with himself. He has to use what he's seen in Edward and Bella to let go of his past enough to be a better vegetarian.
Precisely because death awaits us in the end, we must live fully.
Stars did fly toward each other, irresistibly, as if they were falling in love. And millions of years later, lovers on Earth drew together and fell in love, watching the stars fall.