It's true that sentimentality could pretty much be an illusion, but is that really it?
I became sentimental shortly after the new millenium, and I totally worship the 90's.
Probably I just feel like it cuz I was this happy child back then, but I do think there's a grain of logic in my whiny sentimentality, what with parents who diverced in 2001, school going downhill because of my being lazy and tired of everything, inability to really feel for things the same way and stuff like this.
The loss of strong feelings for everything is very frustrating: eight years ago I could become exalted just from visiting my mother's parents, whom I like very much.
Nowadays it's nothing special at all, I'm just there, and it's over in an instant; time passes too fast.
I actually think you *are* happier during childhood, because you enjoy life in a much more free-spirited way, and you don't yet feel the burden of being an adult and "take responsibility".
You can fool around as much as you like and be judged as completely normal, but suddenly you "shouldn't" do this and that, because it's "too childish".