Like Dark Knight said, it's the freedom from real repsonsibility. Plus, you're so close to being an adult. You get to do all these near-adult things, but without the fear of real consequences. There's probably also the wonder of discovery of one's self--what do I really care about, what am I good at, what am I going to do with my life. That is the time when those big questions start to get answered. And people have such big, idealistic ideas about their future. Looking back, some people forget the details, but remember the general sense of it and miss that.

Now, all that's not completely true, of course. Some people never experience those things and others do it a bit later while in college. It's all individual. It's really a poor general statement, probably most often used by the people, that Tom mentioned, that peaked in high school.

But that's why they say it.