In Ray Bradbury's Fahrenheit 451, why does Montag grin with a "fierce grin" before and after he is fighting fires?

Readers read about Montag's "fierce grin" in the opening paragraphs of the book, and it is very much related to the book's opening line that tells readers that Montag finds burning things a "pleasure."
At this point in the book, Montag loves his job. He likes doing what he does, and it gives him a sense of purpose; therefore, merely thinking about doing his job makes him happy. He can't help but smile when he thinks of getting to burn something or having thought about what he burned earlier in the day. His grin is "fierce" because of the nearby heat and because while he is happy, he is also determined to stand up to the flames. It's a happy, brave, and tough image all at the same time.