I would say...puberty
When we were still a child, we walk around naked without shame..but we slowly feel shame as we grow older. Especially when boys and girls start to have feelings for each other.
Ofcourse, we cannot look down on the influence of the environment we live in because we can never exist alone (screw Robinson Crusoe...its a myth) so religion, culture, social conditioning can also influence this. In the name of civilization.....haha