Final answer:
Dr. Jekyll's sad and despondent demeanor, as well as his refusal to join Utterson and Enfield, is explained by his own internal struggle and fear of turning into Mr. Hyde, as revealed in his confession in Chapter 10.
Step-by-step explanation:
Dr. Jekyll's confession in Chapter 10 of The Strange Case of Dr. Jekyll and Mr. Hyde sheds light on his peculiar behavior toward Mr. Utterson and Mr. Enfield as witnessed in Chapter 7. The confession illustrates the immense internal struggle within Jekyll, his feeling 'eaten up and emptied by fever' due to the fear and dread of his transformation into Mr. Hyde. This internal turmoil explains why he appeared so sad and despondent, and why he refused to join Utterson and Enfield on their walk - Jekyll was afraid of succumbing to his darker side and the consequences that would bring. His refusal to leave the safety of his room was a desperate attempt to prevent another transformation into Hyde and, therefore, safeguard those around him from the danger he posed.