There's really quite a beautiful marriage between Milady's ingenuity and D'Artagnan's immaturity. When they first meet, she's trying to frame him. She's using him for a certain reason. They haven't just met by coincidence. She's singled him out for a reason. She knows that she can almost make D'Artagnan do what she wants to, and that's when D'Artagnan's immaturity comes out.