Classicly

The reasons why that period is envied seem to be these: First, and most subtly underlying all envy of childhood, is the knowledge that it is the time when we have our whole life before us. Often it is not the return of the state itself that is desired, but its anticipation of a life which we feel to be swift and short, and of a past which is irrevocable. Not to be children again, but to have our chance again, is the wish underlying most of the yearning.

Apart from this, there are many other reasons. We may place as the second, the freedom of childhood from responsibility and care; and next, its freshness and its habitual joy; and last, but very far from least, the atmosphere of loving service, kindness, and tenderness which surrounds that helpless period. Of course, we are speaking of childhood under favourable circumstances; no one, except, perhaps, a dying man, would envy the beginning of life in extreme poverty or in loveless hardship.