Theory:

“Earth provides enough to satisfy every man's needs, but not every man's greed.” According to this saying, in this story, the greed of the evil old couple was never-ending. Every time the good old couple got something worthy, the evil old couple destroyed it.
 
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The pet dog of the good old couple was always serving his master, whether he was alive or dead. First, the dog helped to find the treasure for the good old couple, but the evil old couple killed the dog for not finding treasure for them. Next, the dog appears in good old man's dream and asks him to make a mortar from the pine-tree under which the dog was buried. The evil old couple burnt the mortar. Finally, the dog appeared in good old man's dream and asked him to sprinkle the ashes of the burnt mortar on withered trees, and it will blossom. The good old man did the same and was recognized by a prince, who rewarded him with treasure. But when the evil old man tried the same, he got punished by the prince's servants.

At last, the good old couple shared their treasures with the evil old couple and made them realize their mistake.