There’s still a lot that could be better in Won’t Last a Day Without You. For one thing, we could do away with the arbitrary pop song that becomes the title of the picture. It could steer itself away from reflexive references, and use less hokey premises to set up its romance. It could have tried to make fuller characters out of Melissa and Andrew, creating at least some tension in the end. But these details discount the film’s overall charm, it’s winning commitment to romantic pleasures even as it explores the messier side of relationships. Somehow, it just works.