why does Montag think of the old man in the park?

Montag thinks of the old man in the park because he remembers that he was a professor. He wants someone to teach him about the books he's reading, and the old man (Faber) would be the perfect choice.

Montag wishes to have answers to the many ideas that confuse him when he is reading. Faber (the old man) seems to go beyond surface information and thus may be able to help Montag when he feels as if the reading is "like mud."