Why are the controls opposite/inverted when flying in a game?

In a game, we would normally use the directions in a normal way to move, i.e. "up" for move forward, "down" for move backwards, "left" for left, and so on.

Although, when we play a flying game, why are the controls opposite, so "up" is down, and "down" is up, "left" is right, and so on.

How does this effect come naturally when we play it?