Most people here are saying otherwise about that saying and I don't plan on breaking that trend. I suspect that the reason people mention this is because it's the last time in your life where you're not worried about adult things like raising a family, paying the bills, working rough full time jobs, and more. The teenage stage tends to be the first stage where people generally tend to feel "alive" and have much more freedom to do whatever they want. It's also when sex becomes a part of someone's life and by that I mean at least becoming aware of it and since it's new, it feels a lot more wonderful than when you get older. It's even a chance to be a rebel and be like "screw you world!" where you do all kinds of things that people don't want you to do like look up porn sites, play violent video games, flat out have sex with someone, and a lot more. It's like "revenge" for when you were a young child and you feel like you can fight the power unlike before.
I think that's supposed to be the reasons anyway, not something that everyone will experience. If I had to repeat high school, I think I would finally snap and go crazy. Just because life gets harder as an adult doesn't mean it gets worse. For one, you're not treated like crap because you haven't lived for 18 or 21 one years yet. You're also not forced into something like how you tend to be forced into school.
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The world is a very nice place, depending how well you fit in...
The Internet; human stupidity at its finest...