It’s all about energy: What appears to be the bottom-line difference between introverts and extroverts is that social interactions are energizing to extroverts but draining for introverts. This is why I might come to your party but leave long before the conga line starts. And why a stretch of interaction then requires a few days of solitude to recover. If you understand this, you will have grasped a key quality of your introverted friends and their perhaps puzzling behavior (why didn’t she come to the after-hours party?) will make more sense.