That's because you are mostly dealing with how a person feels in the moment, that part of their life that you are around them. Often people do use this "forever" statement because it's how they feel in that moment in time, however, as people move around they lose sight of that and tend to form other relationships. Some of that is how we are designed so that we can keep surviving even if we lose someone close. Life changes us, it's just a part what living our life means and it's gradual. So there can be a memory of that "feeling" and yet as we move forward and change we develop other relationships. It's just how we are designed for our survival as human beings.