He thinks about his previous life and how unhappy and unfulfilled he was.
Explanation:
Earlier in the metamorphosis, Gregor had already shown that he was also a workaholic, quite jealous of his orders and what the company meant to him. Despite hating his boss's actions, he could take it all because he had big medium-term financial plans.
The moment Gregor is metamorphosed and locked in his bedroom, he begins to reflect on how exhausted he was and how unhappy he was in his old life and had not enjoyed his time of freedom to live adventures and new challenges. Thinking about the previous life makes Gregor unhappy and depressed.
