Final answer:
In the story, the characters become anxious in Mirkwood due to the darkness and gloom of the forest, the presence of unknown dangers, and the isolation and unfamiliarity of the environment.
Step-by-step explanation:
There are three main reasons why the characters in the story become anxious in Mirkwood. First, the darkness and gloom of the forest create a sense of uncertainty and fear. Second, the presence of unknown dangers, such as rustlers, adds to their anxiety. Lastly, the isolation and unfamiliarity of the environment contribute to their unease.