What is a "cause of uneasiness” of the family that the monster observes?
In the creature's words:
They were not entirely happy. The young man and his companion often went apart, and appeared to weep. I saw no cause for their unhappiness; but I was deeply affected by it.
As the creature continued to observe those reactions, he identified what he refered to as "one of the causes of their uneasiness." The cause of uneasiness was poverty.
The poverty that the De Lacey family suffered caused them a lot of pain particularly because of their need for food. The creature was very sympathetic of their sadness and was amazed at how the younger De Laceys sacrificed their food rations to be able to feed their father.
The monster expressed his sympathy by noticing how much the family members were willing to do for each other:
Their nourishment consisted entirely of the vegetables of their garden, and the milk of one cow, who gave very little during the winter, when its masters could scarcely procure food to support it. They often, I believe, suffered the pangs of hunger very poignantly, especially the two younger cottagers; for several times they placed food before the old man, when they reserved none for themselves.
As a result, the monster began to feel guilty because he had a habit of taking food from the family storage at nights. Acting as if he were a part of the De Lacey family himself, he also went through a similar period of abstinence in which he was content with finding nourishment in the berries that he found in the forest, all for the sake of helping the De Laceys somehow. This was a way for the monster to feel as if he were one of them, and it was an imaginary way to attempt to feel the warmth of love in his life.