It all starts with Clarisse, that first night that he meets her. They begin talking, and she asks him, "Are you happy?" He insists that he is; but then he gets home, and he finds Mildred and her pills. All of this strikes a chord against the happiness issue; he starts pondering, wondering if he really IS happy.
From there, Clarisse pulls the dandelion trick, and Montag supposedly isn't in love. This, in combination with the unhappiness theme makes Montag wonder. After this, Mrs. Blake happens. She refuses to leave her books in her house and is burned with them. This impacts Montag so greatly that he calls in "sick" the next day (not to mention that he has snagged a book from her house and wants to read it; this isn't new, he has more stashed).
From here he seeks out Faber and is so inspired that they plan a subterfuge by printing books; Mildred's friends so disgust him with their shallowness that it plunges him even further into rebellion. Finallly, after having read some books and being given the support of Faber, the last straw is being called to torch his own house, by his own wife. Beatty threatens to kill Faber, and that's that. Montag defends Faber and runs for his life. He is a full-blown "rebel", but not alone, as Granger and his men confirm. These rebels will be the ones responsible for reshaping their society.