Answer:
This is quite late but some possible internal reason may include...
Internal 1: The teen may be depressed and feel the need to turn to a drug for help.
Internal 2: A teen may be pressured or stressed, using a drug to calm themselves and settle down.
Internal 3: Teens often deal with devastating situations or have been through abuse and use drugs to cope with it.
External 1: A teen may see others using a drug and start to use it themselves to grow popularity.
External 2: Teens often look up to others and will follow their role model's behavior.
External 3: Any teen wants to feel good inside and out, and some think drugs will help them get the pleasure they seek.
Step-by-step explanation:
These examples go for any drugs, really, so I hope they can be useful to those who need help. Remember not to directly copy so you don't get plagiarized, try putting it in your own words.