Why is he having trouble understanding why they are unhappy?

Text Evidence:

"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. If such lovely creatures were miserable, it was less strange that I, an imperfect and solitary being, should be wretched. Yet why were these gentle beings unhappy? They possessed a delightful house (for such it was in my eyes) and every luxury; they had a fire to warm them when chill, and delicious viands when hungry; they were dressed in excellent clothes; and, still more, they enjoyed one another's company and speech, interchanging each day looks of affection and kindness. What did their tears imply? Did they really express pain? I was at first unable to solve these questions; but perpetual attention and time explained to me many appearances which were at first enigmatic."