What could be the reason? Where are his colors?

How do you lose your passion? I used to see my father paint with his eyes glowing during his younger days. He used his brushes with ease, his hand is like dancing in the colors he is picking to paint on his canvas or paper when he is doing his water color. His eyes glow along with his work. His creations were like always like full of perfection and ease. Time went by and he seemed to lose his enthusiasm, he just work on his brushes and seemed unsatisfied with his work. What seemed to be so natural before changed to an effort full work on his canvas. What could probably go wrong when he was for so many years playing with his craft. Now the glow of the colors from his brushes faded for an unknown reason. What could probably caused it? Is it time? Is it age? Is it the materials? A question that probably many artists also experience but for sure not all. What could probably be the reason? Just wondering... until...