A challenge or a sign?

I had a talk with an old friend a week ago. The conversation ended up on the subject of "hitting a brick wall." (It is somewhat related to my previous post.)
When you encounter a set-back of some kind - an experimental flaw, a series of obstacles on the path to a goal, or a huge lack of support from all around you as you embark on a new project - do you see that as a challenge to be overcome, or a sign that the matter at hand is not meant to be?
My friend and I only succeeded in parsing out the problem. No easy formula occurred to us for determining which is which. I guess every dilemma has its own unique factors.
I imagine the age you are when you hit the brick wall is quite significant. Young people are inexperienced in most things, so they will find themselves ill-equipped more often than someone older, generally speaking. The brick wall is more likely to be a challenge. Education can overcome it.
They still have to ask if they should continue taking flute lessons or not, try for the football team for the fifteenth time or change to baseball (or no competitive sports at all), stop pining for that girl in biology class and look elsewhere for love, or take the bar exams for the nth time.
As a writer, I have heard many stories of famous books that were rejected by publishers more than 20 times before someone savvy enought to see the genius in the work took it on - making history. Good example, the 1960s travel memoir The Kon Tiki. How did the author keep trying? Did he ever wonder if the constant rejections "proved" it was a dud? Maybe so - but the determination (or benign sort of egotism) made the author go on, ultimately to success.
That difficulty was a challenge.
To someone else, a sign - not to quit her day job. Being a published author didn't seem to be in the cards. (Giving rise to another potential discussion here about "destiny." No ... I won't go there.)
............
On another note: no word from any of you out there. Four more days and I'm out of here, unless I hear otherwise.