After his first conversation with Clarisse, Montag is moved; he feels alive; he feels happy and interested in the deeper aspects of life. Then, he steps into his own home, Clarisse's haunting question of "Are you happy?" echoing in his head. At first he laughs it off, asserting defensively to himself, "Of course I'm happy." But, as he opens the door to his bedroom, and feels its cold, tomb-like silence, and realizes sadly that his empty wife is there, not waiting for him, not feeling anything, the truth hits him hard. He realizes, sadly, that he was not happy, not at all. As this realization courses over him, the smile that was left on his face as he thought about Clarisse disappears. Bradbury describes it this way:
"He felt his smile slide away, melt, fold over and down on itself like a tallow skin, like the stuff of a fantastic candle burning too long and now collapsing and now blown out."
Any flame of "happiness" that he thought he had in his life, the temporary high of the burning books of his job, the complacent comforts of his easy life--all of this is empty to him now. It does not provide happiness, whereas before, he thought it had. His supposition of happiness, like his smile, fades and is extinguished, and he is left with the dark reality that he doesn't know what happiness is. I hope that those thoughts help; good luck!