In Act IV, Scene 3, Juliet takes the potion. We know that the next morning, her family will find her "dead" and believe she has taken her own life.
In Act IV, Scene 4, we see the Capulets preparing for the big party to celebrate Juliet's marriage to Paris. Her parents are full of excitement and eagerly ordering food and preparations. We know their happiness is about to be shattered very shortly, as they are about to go wake Juliet.
Thus, the emotional impact of Juliet's actions is heightened by seeing her parents' joy and excitement.
