With naturalism, the feeling of despair, and the exposure of the deepest human emotions to intensity are ever-present.
The Joads are at their wit's end. Their boxcar is about to be taken by the flood, the rain does not stop, Rose of Sharon's baby is born stillborn and his body is sent down the river by the current, Pa spends the last of their money, they found that man starving to death, Rose of Sharon breast feeds him with the milk that would have gone to her now dead baby.
That entire situation evokes such an intense set of human emotions and moves the reader to extremes. That is what naturalism intends to do, precisely to unveil the true nature of the extremes and bring them out in words with ample description, and continuous mention of emotions.