The problem isn't the relationship itself. It's just that it can easily ruin your view of a right relationship after you grow up. You will be desensitized over time because at such a young age, it won't be easy to make good decisions. And at the end, the break up is ALMOST always necessary. The break ups aren't smooth and will hurt you enough that you probably won't be the same person for a while. All in all, risking your heart for something that has such little meaning isn't always worth it.