Superstition
Each of the characters of the story are superstitious, but Sam is exceptionally so. When he loses his hand, he starts paying more and more attention to cause and effect. He convinces himself that he can predict the future by the tingling in the stump where his hand used to be. After the Lambs establish their store, they have a series of extraordinarily lucky events which makes Sam suspicious. He comes to believe that they are engaging some magical practices which are influencing their fortune. Considering himself above such things, Sam resists the urge to dabble in the dark arts for only so long. Eventually he breaks down and performs a ritual after which he gets a new job and wins a great deal of money at the racetrack. He would've been smart to listen to Fish and Quick, however. They have been studying the house and become convinced that there's some mysterious curse over the place. With their mom's background in witchcraft -- she brings Fish back from the dead, -- they try to bring peace to the house. This involves a series of interactions with the creepy dark man who seems to be some kind of caretaker. This man is an aborigine who gives subtle hints as to the desires of the spirits inhabiting the house. In the end it seems the marriage of Rose and Quick plus the death of the local murderer put things aright for the house which settles afterwards. Regardless of the characters' various convictions, some supernatural forces seem to be at play in their lives.
Fate
Similar to superstition, fate plays an interesting role in this story. When Sam claims that his neighbors are artificially influencing their luck, he is correctly interpreting events. The Lamb family is focusing on how they can improve their situation which naturally gives them an advantage over their neighbors. It seems to be the case that each of the characters is destined to behave a certain way. Fish is the best example. After his near drowning, he expresses a constant obsession with water. He wishes he were dead, often wandering down to the river to play alone. Whatever the reason, it seems like he was supposed to have drowned the first time. Eventually he seals his fate by jumping in and drowning for real, undoing his mother's spell.
Jealousy
Much of the discord between the two families is born of jealous. There's an innate competition between them after the Lambs open their story. Since the Pickles invited the Lambs to live with them because they needed the rent money to survive, they are naturally envious of the Lambs' financial success with the store. If the Pickles had opened a store instead of renting out their home, they probably would be just as "wealthy" as the Lambs today. Add to this the sexual tension between the couples over Dolly and Lester's affair and you've got a recipe for disaster. It takes a series of hauntings, a murderer, and a marriage to bring them together at last. in the meantime they make each other miserable with their jealous attitudes. Oriel is the main instigator, here, even moving into a tent in the backyard so as not to sleep under the same roof as Dolly. Their differences are petty at best. As the smartest individuals, Rose and Quick seem to realize this best. They put aside their qualms in order to pursue a romantic relationship, intentionally forcing their families to work together.
Inheritance Tied to Location
The Pickles' home is Sam's inheritance from his parents. When he moves in, he realizes all too late that some weird energies are at work in that house. People have died there. And there's some strange secret aboriginal lore connected with the place, probably leftover from the days when orphan native girls lived in a kind of orphanage there. Whatever the cause, these spiritual forces are inexplicably linked to the house itself. Their energies manifest in the house for the few who are observant to notice, like Fish. In the end, Sam realizes that he must stay here on Cloudstreet. He must make good his parents' investment in him by trying his best to bring peace to the family, which eventually is true.
