Romantic relationships, especially in the teenage years, often affect the mental well-being of individuals (both negatively and positively). For example, if someone was the victim in an abusive relationship, he or she would be more insecure and skeptical of peoples' actions. Contrastingly, it is possible for an individual to be emotionally "built-up" by a healthy romantic relationship: he or she may be reassured of his or her attractiveness and become more confident.
That's a good thought. A lot of growth, both emotional and physical, takes place during the teenage years, and I also think that relationships with people you really care about play a large role in shaping this emotional and social growth. Relationships can affect how a person sees themselves, and this can either stimulate or deter the building up of self-esteem.