Fear is the mind-killer. Wait, that's from Dune. For Dr. Seuss, fear is the friendship killer. In "What Was I Scared Of?", the narrator is afraid of a pair of floating, empty, pale green pants simply because they walk around on their own. In the forest. At night. Actually, that fear makes a lot of a sense. Still, when he finally gets to know the pants (odd as that may sound), he discovers that the pants are afraid, too. Imagine that.
Q: What is a pair of empty pants doing walking around the forest at night?
A: They probably have errands to run, just like the narrator. The two seem to have a lot in common.
Q: Why did the narrator hide in the Brickel bush if it was so uncomfortable?
A: Fear will make you do some pretty silly things sometimes. Best to stand up to your fears if you don't want brickels in your britches.
Q: Why does the narrator keep running those errands if he's so scared?
A: Sometimes you've got to do what you've got to do. Judging by the look on his face, he must have really needed those Snides.