Sometimes giving it a second chance is worth a shot, and other times, the third time may not be the charm. While some celebrity couples, such as Kate Middleton and Prince William, have proven that a break is necessary to strengthen a relationship, others, like Selena Gomez and Justin Bieber, probably should have made a clean break the first time around. From Elizabeth Taylor and Richard Burton to Pink and Carey Hart, we're taking a look at all the celebrity couples who've been on and off again so much you'll feel like you're riding a roller coaster.