A common reason teens use drugs is to appear cooler, and fit in. This is insanely dangerous, as it creates the mindset that if they do a certain thing, it'll make them popular, which can lead them to even death.
Another, more saddening reason, is that teens need an escape from reality. Their home life might not be good, or their school life is bad and they get bullied. However, turning to drugs should never be an option, because reliance on these drugs for comfort leads to severe addiction.
More times than not, teens end up using drugs because of family members. Parents, siblings, cousins, or even family friends can easily influence you, because of the trust you have in them. This is why it's important to know how to say no, and how to recognize the signs. You should always look for help when someone close to you tries to get you to do drugs.
Believe it or not, parents who over-protect and over-restrict their child only makes the child more rebellious. The same thing works for under-protective parents. This can be prevented by letting your child be indepentant, but still lecture them of right and wrong, and the dangers of things, such as drugs.
A final reason, of the many that teens do drugs, is simply curiosity. The urge to want to know how being high feels, how the first puff feels to exhale. However, this can lead to the serious consequence of addiction, and eventually death, if the line is crossed.