Why do the Japanese do this?

[Roto13]

Because it's part of their language. A better question would be "Why do American teenagers do it?"

It's not just "American Teenagers" Plenty of Non-American Adults and Non-American Kids do it (Not to mention Non-American Teenagers)

Adults who actually act like adults don't do that. :P

But yeah, I probably should have said something like "non-Japanese teenagers", since asses know no borders.