Many hobbyist's get fish that in the wild may live in faster moving streams or rivers...so they attempt to simulate this condition in their aquarium (which can be difficult to do). But often, many of these fish were bred and raised in tanks or ponds and never saw fast moving water. Imagine the difficultly of being tossed into a new environment completely unlike what you've grown accustomed to. Talk about a recipe for stress.
In general, we want good circulation in an aquarium to minimize thermal gradients and to somewhat evenly disperse minerals and nutrients. But excessive flow is often counter productive and in some cases, potentially harmful.