Parents in denial. Parents find it difficult to tell children not to do what they did themselves. Youth are not fully aware of the consequences. Other personal problems are the real issues, not the drugs. No other alternatives to cope with their personal problems.
Youth feel it's all part of growing into adulthood, see adults doing it.
Mixed messages from adults and the media. Sometimes drugs are bad, but okay for medical purposes. Substance abuse among younger and younger youth. Easy access and drugs are seen everywhere. Students come to school high because they think it's cool.