Teenagers are, by definition, not always eager to share personal information with their parents. They are at a stage in their lives when they are working hard to figure out their own independence and identity. Part of establishing independence may include doing things their parents don't approve of. Another part of exploring identity has to do with trying on behaviors sanctioned in their peer culture. Parents are, by definition, deeply invested in their children's lives.
Parents hold expectations, values, hopes, plans for their children.
Confronted with children who are testing out their own values, expectations and plans, parents often feel scared, disrespected, confused and powerless. Effective communication is difficult under these circumstances. Keeping your children safe, establishing two-way communication, figuring out your sphere of influence and dealing with your feelings are challenges facing many parents of teenagers. When they were younger, we discovered that we really couldn't control what they ate or how much they slept. When they went to elementary school, we found out that we couldn't control who they made friends with.