The generation gap is caused by the conflict between the life experience and responsibility of older generations the idealism of the younger generation. As we grow older and assume more responsibility, our goals change. We seek safety and support for ourselves and those we are now responsible for.
Ironically, the generation gap is most easily seen between parents and their kids. Having children profoundly changes one's outlook. We want our children to be safe and successful and that makes us push them in those directions. Children, even into young adulthood, want to explore and learn and experience new things. Often these new things do not contribute to their safety and success, thus the friction between parent and child.