When people describe themselves with “I am”, they are also sharing something they identify themselves with. “I am” is a way to share their identity.
With “You are” people describe how they see the other person’s identity. For the recipient, this can mean that he is defined in a different way than they see themselves. That’s a moment in which they can feel threatened in their identity.
Those seeking to learn from a teacher or another authority figure can even feel defined by it. It takes guts to believe one can achieve something after being told that you can’t.