Peers may feel that they want their teen friends to drugs with them, this can be harmful because the victim may not be able to say no.
Feeling "grown up" may be what teens want, they may see adults doing drugs, and they may want to feel like an adult.
Teen curiosity is a big problem, they may feel that they NEED to try it, to feel like they fit in or are part of the group, they may just want to feel it.
Rebellious teens may want to get back at their parents, their family, or just life, they should take their anger out another way besides drugs, or alcohol.
Lastly, a teen may NEED drugs, because they have become addicted, and can't stop themselves. This is when it is dangerously bad.