Even the worse humans have dreams, hopes, and plans. In The Martian Chronicles, every major character has some goal to increase his happiness (or avoid unhappiness): Yll in "Ylla" is trying to prevent his wife from leaving him; Captain Jonathan Williams wants to be celebrated for reaching Mars ("The Earth Men"); Benjamin Driscoll wants to plant trees to increase the oxygen of Mars ("The Green Morning"); and even the automated house of "There Will Come Soft Rains" wants to keep up its routine. Humanity is a race of dreamers, even when there are only a few of them left.
In The Martian Chronicles, characters often fail to achieve their dreams because the world is constantly changing. A character may momentarily get his wish, but it doesn't last.
Although dreams can be hard to achieve, they are both achievable and necessary in The Martian Chronicles. A character without a dream cannot set a story in motion.