Teenagers commit suicide for a number of reasons, almost all stemming from deep bouts of depression. It is the underlying cause of that depression that mental health experts seek to address.
Many teenagers who commit suicide do so because of an overwhelming sense of hopelessness regarding the future. Planning one's life out, or even contemplating it, can be emotionally overwhelming for many teens, especially as they edge closer to completing high school. The responsibilities of adulthood, including holding-down a well-paying job, raising children, paying bills, and so on, all become too much for some children to bear. They may harbor resentment against adults in general for some real or imagined grievance, or against their parents for reasons having to do with their upbringing.
Teenagers also commit suicide over despondency following a particularly traumatic act or series of incidences. Teenagers who are being bullied may commit suicide because they fell humiliated, weak, and alone. They may commit suicide because they have been sexually assaulted. Drug and alcohol abuse may be a contributing factor, or the result of incidents in question, for example, a history of child abuse at the hands of parents or guardians. Girls are statistically more likely to attempt suicide than boys, due in no small part to the history of relations between the genders, the propensity for girls to be sexually molested (which, of course, happens to boys also), to feel peer pressure differently, and to feel depressed about their physical characteristics. Despite the greater likelihood of girls attempting suicide, however, they are less successful in doing so than boys, who "use more lethal methods, such as firearms, hanging, or jumping from heights." [kids.health.org/parent/emotions/behavior/suicide.html]
Failure by parents, teachers, and other adults in a teenager's life to detect signs of depression and possible suicidal tendencies is a problem unlikely to go away, given the assumption among many parents that rebelliousness is a part of growing up.