Why did Montag say he wanted to become a fireman?

After Montag meets his intuitive teenage neighbor, Clarrise McClellan, he begins second-guessing his occupation as a fireman and starts to analyze his meaningless life. On a routine fire call, Montag witnesses a woman commit suicide with her books and refuses to go into work the next day. Montag becomes completely jaded with his occupation and regrets becoming a fireman. When Mildred mentions that he should have thought about it before signing up to be a fireman, Montag says,

"Thought!...Was I given a choice? My grandfather and father were firemen. In my sleep, I ran after them" (Bradbury, 25).

Montag's response is significant and indicates that he never had a choice regarding his future occupation and simply conformed to his family's expectations by following in the footsteps of his father and grandfather. Montag comes from a long line of firemen, who supported the destructive institution and loyally served the authoritative government. As a child and adolescent, Montag anxiously awaited the opportunity to one day wear a fireman uniform and never thought about becoming anything else. Essentially, Montag was inspired by his father and grandfather to become a fireman. Despite his enthusiasm as an adolescent, Montag is beginning to experience a dramatic transformation and question his entire existence.

Montag came from a legacy of firemen. His father and grandfather were both firemen, and so he became one too. For many years, Montag enjoyed his job and was known for doing it well. However, after encountering the free-spirited Clarisse and witnessing the old woman who chooses to burn with her books, he rapidly begins to question his government's policies related to censorship and the limitation of free thought, as well his own role in enforcing those policies.
When he reveals his evolving perspective to his wife, she retorts that he should have thought of all that before becoming a fireman in the first place, and he says "Was I given a choice? My grandfather and father were firemen. In my sleep, I ran after them." Essentially, he never really "wanted" to become a fireman. In his highly conformist society, he just blindly entered the profession and fulfilled its duties because it was what was expected of him.