Sometimes in a situation like yours two people can form a relationship young and they actually both continue to develop because our brains don't fully mature until around age 25. Often what can happen is once the individual matures and gets involved in working and engaging more of the adult world, they may suddenly have an unfulfilled feeling they don't quite understand. It can be a strong desire to fly solo for a while before committing to a family life that will most definitely tie them down more.

Sometimes, sadly, a couple can actually grow out of their relationship or a partner may feel they have. You have been together from such a young age, sometimes a partner never got to experience real freedom and they begin to wonder what that might be like.