"Those that seek the easy way do not seek the true Way." - Dogen
The fact of the matter is that training in the martial arts presents challenges. There is no such thing as a prodigy. No one wakes up or is born just "knowing." Each of us has to put in the time, energy and work - no one is immune.
When we are presented with a situation that is difficult or challenging we have two ways of looking at it. The first, and healthiest way, is too see it as an opportunity. The second, and less healthy way, is to see it as some type of burden or as a oppressive catastrophe.
Challenges help us change. Nobody ever improves themselves by playing it safe.
So, it is not that change needs to come by way of challenge. It is just how we are wired as human beings. When things are good, we tend to not look inward. There is an old sports saying that is pertinent, "Sometimes you win, sometimes you learn."
Warriors are people who deal in change and their currency is challenge. To improve anything, one needs to first look upon a challenge as an opportunity for growth and ultimately necessary for change. Warriors of old understood that adversity is the necessary building block of character.
Change requires challenge and challenge causes change.