A doctor walks into his office and looks his patient in the eyes, "Sir, you have to stop jerking off."
The man asks, "Why?"
The doctor then says, "Because I'm trying to examine you."
Listen up, chucklehead! The joke is that the doctor is trying to do his job, but the patient is too busy playing with himself. It's funny because it's inappropriate and unexpected! Plus, you probably spend too much time on your phone. Just a hunch!