The morning that Montag decides he is too sick to go into work that day, it is right after he watched an old lady burn to death with her books. This was incredibly upsetting for him; he realized that this woman cared so much about her books, and about what was in them, that a life without them was not worth living. This strikes right at the center of a conflict that Montag has been facing for the past few weeks. He has realized recently that he is not a happy person, and that his wife isn't happy either. He has realized that he is missing out on what life is really about, and that his life has become empty and meaningless to him. Clarisse, who he's been talking to quite a bit, is so full of life, vivacity, curiosity and joy that it is a stark contrast to how he feels about life. He realizes that there is nothing in his life that he cares about so intensely as Clarisse cares about things, or as Mrs. Blake, the woman who died in the fire, cared about things.
These realizations, that he is not happy, and that he is completely clueless as to the real meaning of happiness and life's value, are crystallized as the old lady dies in the fire, and as Clarisse "disappears." He feels empty and alone, and his job, which used to bring him a fierce kind of happiness, no longer holds appeal to him. It is just burning up something that he feels holds the answers to his questions--books. He can't do it anymore; his heart isn't in it and he is needing a break to try to figure things out.
I hope that helps a bit; good luck!