I never understood that above argument. Of course "things were better because you were young" would apply because, typically you did not have a lot of responsibilities then. Go to school, do your chores/homework, make friends, learn from your mistakes.
The good times from childhood would be from your favorite entertainment fixes, the memories from holidays/birthdays with family, the friends you made along the way, learning how the world is day by day. I think it also relates to two things: the wave of technology and Internet was rapidly setting stage for millennials in the 2000s. Everything was feeling new and improved; HDTV, smartphones, online multiplayer, social media.
It's easy to reminisce about childhood, good and bad, because you have hindsight to learn from before to not repeat itself. Again, the 2000s, even after 9/11, had promise for prosperity and growth for Western civilization thanks to advancements in technology and overall quality of life improvements compared to prior decades. As proven now, greed, power and entitlement whisked away those elements of promise for that generation and the next.