Sometimes people have to learn things for themselves. It's just like when you tell a child not to touch the fire because you'll get burn. You can tell them that over and over but eventually they touch it anyway and then they see what you mean and won't touch it again because they remember what it felt like when they touched it.
I know at 14 and throughout my teenage years I thought knew what love was, regardless of what the older crowd told me, until I grew up and smelled the coffee.