Most of Montag's society suffers from illusions of happiness. Through keeping them super busy, pumping them full of useless information, and giving them outlets for violence and frustration, their society "funs" them right out of happiness, while making them think that they are happy. Take Mildred's friends for example; they appear to be happy, but it is just an illusion. They brush negativity off instantly. When Montag brings up the war, Mrs. Phelps optimistically says of her hubby that just got called to fight, "He'll be back next week...I'm not worried...I'll let Pete do all the worrying." and then she giggles. She says that her and Pete promised not to shed a tear if something happened, but to go right ahead and get married again. So, cover up any misery with more activity.
When Montag reads them the poem, it wipes their illusions away; Mrs. Phelps is left "sobbing uncontrollably" and Mrs. Bowles gets angry at Montag for reading "hurting words". They can't stand the thought of anything unhappy; when forced to think of it, they get angry, break down, and try to distract themselves, all to promote the illusion of happiness.
It is just an illusion though; if it was real, why would Mildred repeatedly overdose, or Clarisse's friends kill each other so often? You can only fool someone for so long; misery can't be kept out at all times, and when it hits them, it hits hard.