Here's why:

* Fire: Fire is associated with heat, intensity, and vibrant colors like red, orange, and yellow.

* Sky: The sky at sunset often takes on these same colors.

By comparing the sunset to fire, the metaphor emphasizes the:

* Intensity: The vividness and brightness of the colors.

* Transience: Just like fire, the sunset is temporary and beautiful in its fleeting moment.

The metaphor helps us visualize the beauty and spectacle of the sunset in a more dramatic and evocative way.