Why is because he can't read body language or verbal tone.
Go on the internet and look up social stories. They would be good for you to do with him.
Also, watch a movie he likes with him, stopping the action every so often to explain the body language of the actors ie (See how he keeps looking away? That shows he wants to escape. See how she is tapping her fingers on the table? That shows she is nervous. See that frown? That is a frown. It means he is angry. You can tell that is a fake smile because the eyes don't match the mouth. etc.
He can learn to interpret some body language, but he needs you to point it out for him.