Final answer:
Darcy didn't want to ride back with Tarah and Cooper because he was tired from a long trip and feeling sad and preoccupied.
Step-by-step explanation:
Darcy didn't want to ride back with Tarah and Cooper because he had returned from a long trip and was extremely fatigued. He had traveled 120 miles, including a lot of climbing on foot, in just three days.
Additionally, Darcy was in a sad and preoccupied mood, possibly due to the loss of Jane's white herd and the precarious condition of her fortune.