People get involved with drugs for many reasons—seeking a new thrill, escaping from a problem, rebellion, boredom and so forth. The same is true with alcohol.
But you're right: Many young people get involved in drugs without fully realizing what they're doing. Teenagers want to fit in with the crowd—and if their crowd is experimenting with drugs, they may give in and experiment also. Peer pressure is a very powerful force at that age, because they don't want to appear odd or out of step. But many illegal drugs are very addictive, and before they realize it, they're hooked.
This is why I urge parents not only to talk to their children about drugs, but also to warn them about the dangers of following the wrong crowd. The Bible says, "He who walks with the wise grows wise, but a companion of fools suffers harm" (Proverbs 13:20). It also warns, "Do not be misled: 'Bad company corrupts good character'" (1 Corinthians 15:33).
Pray for your daughter, and let her know that you love her and want to do everything you can to help her. In addition, urge her to put her life into Christ's hands, and to look to Him for the help she needs. God loves her (and you as well), and when we have Christ in our hearts we can face the future with hope.