My Heart Will Go On - Celine Dion (The theme from Titanic)
This is another one of those love songs that ends very badly, but it's popular and compelling and doesn't actually involve a breakup. After all, it's not the guy's fault he froze to death. The song is still a great testimonial to the power of undying -- well, transcendent love, right? Wrong. He dies. It's sad. No sad songs for your wedding.