Indeed, this is something I have found to be true as well. To add to your point, when preparing to explain the reason I have to turn down a request I find that it forces me to reevaluate. Sometimes I find that the request is actually reasonable and my real reason for turning it down was only an emotional reaction to feeling otherwise overwhelmed rather than a problem with the request itself.

In those cases I may still turn it down but often I find that I can make a compromise at that point that satisfies both parties.