Because you can't help but stop and notice the reddish orange twilight sky - paying attention to the sunset doesn't take any extra work or cognitive control - our attentional circuits are able to refresh themselves. A walk in the woods is like a vacation for the prefrontal cortex.

Strolling in a city, however, forces the brain to constantly remain vigilant, as we avoid obstacles (moving cars), ignore irrelevant stimuli (that puppy in the window) and try not to get lost. The end result is that city walks are less restorative (at least for the prefrontal cortex) than strolls amid the serenity of nature.

The person who wrote this wouldn't last long in the wild.