Proprioception.

Your mind knows where your hand is at all times and will tense up against the pressure you apply.

When someone else massages you, your body does not know exactly where there hands are and so will keep the muscles relaxed.

It's mostly genetic. Body language and facial expressions are largely programmed into our DNA and we are born with the knowledge of how to perform them and interpret them. This is unlike verbal language, which is 100% learned.

Have you ever noticed that no matter who the person is on this Earth, whether they be Chinese, African, Native American, European, etc, you can kind of tell if they are happy or sad from a picture or drawing without ever needing to learn anything about their language? This is pretty remarkable. We have a mechanism of communication amongst humans that is universal and belongs to us as a species.

Dogs and other social mammals also have body language cues they use to communicate with one another. You can look up a handbook of all the various dog body language meanings. Tail wagging indicates they are happy or anticipatory, raised teeth and ears back indicates aggression, etc.