A really good way I've heard it put is that introverts expend emotional energy on social interaction, and recharge it through isolation. Extroverts are the exact opposite. So if you imagine being forced to interact with other people non-stop for seven days, with no chance to have any alone time, and that's what this person was putting themselves through.

This metaphor also handily explains the difference between "introvert" and "antisocial." I love being around my friends, in the same way that some people love swimming. But just like swimming it's very tiring. Even Olympic swimmers can't swim all the time. So it's not that I don't like you, it's just that I'm exhausted and if I keep going I'll drown.