If by internal reasons you mean mentally, a teen might be feeling rebellious or depressed. Some type of mental illness would contribute to the decision to take this drug, or any drug in general. On the other hand, being a teen, they could be peer pressured into it or just be curious without knowing the dangers of heroin use. External factors? There aren’t any. Heroin makes you high and wonky, but there are no health benefits. Perhaps they heard a misleading comment that heroin makes you skinnier, therefore would take it for a better body image.