Why does Montag grin with a "fierce grin " before and after he is fighting fires?

Montag grins a "fierce grin" because of the pleasure he experiences in burning the books. He enjoys it.

IT was a special pleasure to see things eaten, to see things blackened and changed. With the brass nozzle in his fists, with this great python spitting its venomous kerosene upon the world, the blood pounded in his head, and his hands were the hands of some amazing conductor playing all the symphonies of blazing and burning to bring down the tatters and charcoal ruins of history.

Montag grinned the fierce grin of all men singed and driven back by flame. He knew that when he returned to the firehouse, he might wink at himself, a minstrel man, burnt-corked, in the mirror. Later, going to sleep, he would feel the fiery smile still gripped by his face muscles, in the dark. It never went away, that. smile, it never ever went away, as long as he remembered.