It occurred to me, upon reflection from the safety of my office chair, that the horse isn't particularly happy in those moments, either. He's lost harmony with his closest herd member--me, the rider--and one thing a horse wants is to get along with the herd. It's that survival thing again. The reasons for this unhappy situation can be myriad. That's not the point. The point is, we've gone from a place where we're all having fun, to a point where none of us are. We're each solely concerned with--that's right--our own survival.
It's a lot like life. There is joy and freedom when Go and Stop are both options, and when none of the participants are in survival mode. When we keep an eye out for each other, and stay sensitive to loss of balance, Go is bliss.