Logically, Harry knew he shouldn't have said or done anything because this would definitely blow up right in his face, but Harry wasn't logical. He was impulsive.
Harry Potter, all his life, believed he was alone.
That was before he met his twin sister, of course.
Giovanna Potter, all her life, believed nothing could distract her from school and family
That was before she met Draco Malfoy, of course.