Montag becomes a fireman because it is a prestigious job in his society. He feels honored to be a fireman. For ten years, Montag enjoys his job, feeling strong and powerful as he watches books burn. His position allows him a degree of freedom, he gets to see the forbidden books that are being burned. It makes him feel important. It is the most exciting aspect of his life.
"Montag comes home after a satisfying book-burning, only to find that his house feels like a "mausoleum" and his wife "cold" and himself "with the feeling of a man who will die in the next hour for lack of air."
It is actually Montag's destiny, it's as if his physical genes have destined him to becoming a fireman. Every fireman, when he begins to think about it, has soot colored hair, dark eyes, and a heavy shadow of beard by the end of the day. It's as if people are stereotyped into their life long professions based on their physical appearances which is just how this society acts- everything is superficial. People do not have any real experiences, they only have simulated experiences. By simulating everything, including the illusion of happiness and contentedness no one becomes upset or depressed anymore, unless and until they find the time away from the TV walls and other distractions of life to really sit and think- then they realize how worthless their existence actually is, which is why there is such a large number of people attempting suicide. It is so large in fact that the society anticipates it to the point they have a machine and technicians to quickly fix these lapses in simulated happiness.
Montag became a fireman because he follows his family tradition, for his father and grandfather were also firemen. This ties into one of the themes of the novel - the danger of just following along. This too is what has gotten the society of Fahrenheit 451 in so much trouble. People have stopped thinking critically. Instead they just follow along. They allow the parlors to keep them entertained. They allow the government to dictate what defines worth and happiness to them.