I have a theory, about introverts.
You know people, I'm sure, who can happily be around others for hours without burning themselves out. And then, you know others who can feel exhausted after even a short bout of interpersonal interactions. You may even be one of those people.
There are many possible reasons for why someone could feel consistently tired or drained after being in social situations. It can take a lot of brain energy to track all the shifting social dynamics and power structures. If you deal with anxiety it can take an enormous amount of energy just to 'act normal' (whatever that means) and keep it together. Sometimes you're just tired, and you might be pushing past that fatigue in order to be social.
My theory is that, for some people, part of the fatigue comes from what their face does when they are socially engaging.