Why are some teen girls so stupid?

I'm a teenage girl myself, I'm 16. I've heard countless stories, some even from my school, of girls my age who do stuff like start self-mutilating because they're just sooo in love with some guy who "broke their heart," or intentionally getting pregnant to keep a guy in a relationship, and just so much other dumb sh*t. Obviously, I know not all girls are this way, but why are some? Really. Do they know anything?