I'll offer two theories: 1) Introverts may have a higher default level of arousal, so that social situations overstimulate them and make them tired. In this ca

I'll offer two theories: 1) Introverts may have a higher default level of arousal, so that social situations overstimulate them and make them tired. In this case extroverts would be the opposite, i.e. their default arousal is lower, so they seek social situations to raise their arousal level. 2) Introverts process social situations more deeply, and therefore expend more energy in the same social situation than an extrovert would.