I went on a hike a few months ago that was basically an entirely uphill climb. There were no real twists or turns, just a straight incline. After about an hour on continuous climbing, I just gave up. This was partly because I was exhausted, but also because I had no idea how much further it was to the top. There were no clear indicators of progress, nothing to feel good about, just more turns of the road, and nothing but up to look forward to.
David Churchville / Agile Project Planning Blog
The Prime Directive of Agile Development - Never Be Blocked (Bob Martin)
http://butunclebob.com/ArticleS.UncleBob.ThePrimeDirectiveOfAgileDevelopment