While "dim" might not've been the right term to use, Deimos was definitely a little slow in the free-thinking department. Aphrodite would've encouraged it, if he spent more time with her, but Ares believed in a more orderly way of doing things, in that Ares gave the orders, they were followed, and there was no touchy-feely hand-holding.
When Zeus mentioned Love, it did make Deimos think. Of his mother. How much he missed her, and that he really did love her, it just wasn't encouraged that he be open with it. He'd barely tuned back in to hear, Is fear not also an aspect of love? and he sort of tuned back out again as he mulled that question over.
He sincerely hoped it was a rhetorical question; Deimos and his twin Phobos had both been nurtured from birth with the idea that fear was the building block of everything. There could be no advancement, on the battlefield or otherwise, without the fear of stagnation. There could be no heroism without the fear of cowardice, no love without the fear of being alone forever. Had Deimos been older, wiser, or more familiar with the free thinking Zeus was trying to encourage, he might have been able to argue the concept of duality and the idea that fear was a component of nearly all relationships to some degree.
Deimos really didn't hear Zeus' other questions, because he was too busy still working out the first one, and though he wasn't actively aware of it, Deimos' mind was starting to turn corners, opening up like little buds here and there as he considered new things.
When Deimos did finally snap back to what was going on in front of him, he saw that Zeus had finished talking and was looking at him expectantly. He had no idea how long he'd zoned out, or how long Zeus had been waiting for an answer. Apparently, not long enough for him to lose patience, which for Deimos was a good thing.
"Fear is... necessary," Deimos finally answered, trying to put into words what he'd been taught from birth. "It's a part of every interaction that happens between beings, even us Olympians," he said slowly, marshaling the right words that were unfamiliar on his tongue. "It's what drives things, it makes things happen. Ares has this idea, a war that needs to happen, and so me and Phobos go to the city-state, stir up the masses, inject a little fear and panic of the other city-state, and it drives the people to make war on their neighbor. And on the smaller scale, it drives what happens between people. But that is what occurs naturally, and there's not much that we can do about that."