There is a real pleasure in flowing with someone your own level or higher, someone that can push you just past where you are comfortable yet keep the continuity going to explore more.
Obviously training with Sonny was like that, and if you ever see any video of me training, I'm often grinning .... apart from the times when he jumped a level, and then my mouth gets about as wide as my nose as I concentrate on keeping my focus and trying not to freeze or flail.
I always wondered how HE did it though - how did he keep interested playing with students all the time?
I don't think there was any of us that could give him a run for his money if he wasn't playing, and even when he was, he pretty much always chose when he let you get in. (After his brain surgery, in the last few months of his life I remember playing, him standing in the center of the room in house slippers and armed with a Sundang, and me circling, trying to hit him. I was kinda astonished that I couldn't get past his defense. I couldn't fake him out to make a big enough opening I could take without getting my fingers cut off or being impaled on his blade. Made me realize how much more he had left to show us.)
So I started watching him teach, and after watching him for many years flow with others I realized what he did was to handicap himself, and this is how he kept himself amused when he worked with people with lesser skills.
He was always looking for how slowly he could move and still get away with stuff, how little he had to move his feet to evade, or tilt his body or weapon to gain the advantageous angle. He was also looking at how each student could be 'played' .... This was a huge part of his entertainment, but also of how he kept improving his own skills over time. He learned from everybody that stood in front of him, and I am grateful for that lesson too.