I think teens feel they can't relate to their parents because it's hard for them to think of their parents at their age, having the same dilemmas they currently face. They think it was easier for their parents and that their parents cannot possibly understand what it's like to grow up now.
Fortunately, there is a solution to all this. That is, parents should talk openly with their teens about issues such as drugs, sex, and peer pressure. When the parents are the first to bring it up, it actually alleviates some of the pressure the teens feel with bringing something up to adults that they themselves might not yet be comfortable with confronting. My mother was like this with me and my siblings and that made it that much easier for us to relate to her.