Because we're still young and innocent :)

it is full of joy and happiness

don't compete for everything such as ?

At that time, you aren't concerned in earning money. You needn't think a lot. You make many good friends. They don't compete with you for everything except the studies.

I think its reasons are: we can learning more in schools,find some friends here and get a good growing environment.Many people think that school time means youth,hapiness and no trouble.In fact,when they have been in society,they have found many things that they have never seen and they don't want to see.So they will recollect their youth,their school's lives.They think it's the best part of their lives.