Teenagers can be rather immature, because they have not gained enough life experience to make proper and informed decisions, especially about relationships. At this age range, they are also more prone to curiousity which might lead them to make ill-informed decisions which they might regret in time to come. They will soon mature and outgrow all the things that they did when they turn into adults, and this might include their relationships.
I believe that teenage relationships are more about discovery and curiousity rather than commitment and companionship. Most of the time the teenagers are still figuring out who they are and do so through their relationships. Soon their tastes change or ideals and mindsets evolve and they realise that they might want to move on to another phase of their life without the teenage sweetheart. Sometimes though teenage relationships do go the distance.
Teenagers are new to romance and are therefore doomed to operate without the benefit of experience or the wisdom that goes with it. In addition, their bodies are full of hormones that are new and usually unbalanced. That will change in time, but while they’re still undergoing puberty, teens tend to feel everything very acutely, which often leads to drama. One thing that can make any relationship difficult is when one or both parties have self-esteem issues, and teenagers are notorious for having feelings of insecurity. When you add all of these things together, it’s no wonder why teen romances tend to be short-lived.