There is a belief that a prenuptial or premarital agreement is in some sense asking a marriage to fail, or else it shows that the two people marrying are sure it won't last. After all, why else would you sign a contract before marrying that will only take effect if you get divorced? If everything goes well and the married couple sticks together, there's no need for a prenuptial.
But this question can be answered with another question: why bother buying home insurance, or auto insurance? If everything goes well, you're just wasting your money. Why buy health insurance when you can just stay healthy? And why save up for your retirement when you can just keep working until you die?
The future is unpredictable, and a lot of terrible things can happen that we don't like. Some of them we can't help, and some of them are ultimately our fault. But we shouldn't punish ourselves for making a mistake or for not realizing what could happen, not when it's possible to prepare for a bad future and reduce the harm an accident or a bad decision can cause. We all like to think we'll always make the best decisions, and most people make more good decisions than bad, but you only need to mess up once to cause a big problem.
A prenuptial agreement is an extension of the old saying, "hope for the best but prepare for the worst." Everyone would like to think a marriage will last forever, but the fact is that half of all marriages end in divorce. People make mistakes, people change over time, and going by the average marriage age and life expectancy you'd be spending your life with one person for 50 years. That's a long time and more than half your lifespan.
When you marry someone, you should want that marriage to last. You should want to be part of the lucky half who never fall apart. But you also need to realize that you can't control the future no matter how much you want to, and you should prepare for the worst by ironing out who gets what beforehand.
A prenuptial agreement can make a divorce much quicker and more painless than it would be without one, making it easier to move on with your life afterwards and very possibly letting you and your new ex get along instead of growing even more bitter. Just like your home insurance, there's a good chance you'll never get to use it, but getting insurance doesn't guarantee your house will burn down and getting a prenuptial doesn't predict your marriage will fail.