A common characterization you can find on the Internet of introversion and extraversion is that introverted people consume their energy when they're around other people (and therefore need alone time to recharge) whereas extraverted people consume their energy when they're alone (and therefore need to hang out with others to rejoice). Here I suggest a more sophisticated theory in which both introverted and extraverted people consume their energy when interacting socially, but the exhaustion only shows in introverted people because of other circumstances.

First, I don't think that introversion-extraversion is a continuum, but rather two independent traits. Introversion is the ability to find value in alone time, whereas extraversion is the ability to find value in social time. Those two abilities can be balanced, which arguably constitutes healthiest type of personalities. Or there can be an imbalance between the two traits, which yields "introverted" or "extraverted" personalities, to some degree according to the value of the imbalance.

So, whenever you try to hang out with introverted people, you're actually competing with their own alone time, and they're calculating whether the value they'll get by hanging out with you will be worth the value they get when they're alone. On the opposite, extraverted people are on the lookout for social interactions, because that is a sure source of value to them. There is a sort of belief that extraverted people have an easier life than introverted people because they have greater social support, networking, etc. But that is only as true as there are opportunities to socialize, which is a resource that you sometimes don't control (I'm looking at you, 2020). In other words, introverted people are better at handling solitude.

Naturally, because of their inclination towards alone or social time, introverted and extraverted people will develop opposite habits of socialization. Socialization is a different beast entirely. Socialization is like a muscle. Like any muscle, the pressure it can handle comfortably depends on how much it has been worked out. Like any muscle, it is healthy to exercise, and all sorts of feel-good substances are released when one exercises it. Like any muscle, it suffers fatigue from exercise that is long enough, or that is harder than it can handle.

Because of their social habits, introverted people tend to have an under-developed socialization muscle, whereas extraverted people tend to have a well-developed socialization muscle. Now, since extraverted people are the one running the social show, most standard social situations in life range on a level that is above what introverted people have been trained to handle. Their muscle is being over-worked and the energy wears off rapidly. In extreme cases (e.g. a shy person being catapulted into a big party), the muscle is being burnt out so violently there is no possible enjoyment and not even productive workout going on. On the other hand, extraverted people are simply putting a healthy dose of exercise on their muscle, which triggers liveliness in them, because that's what happens with healthy exercise.

That is the reason why we say that introverted people are exhausted by social interaction whereas extraverted people are energized by it. This is not a good definition of it. This is merely an observation of the consequence of it in some specific circumstances. The point being, introverted people can also energize with social interactions, only if they can find a social setting that is appropriate for them; and extraverted people can also get exhausted by socializing, if they abuse it and forget to add distance to have a rest.