LOL My dressage horse is too lazy to want to break into a gallop - nothing would make him more miserable!
The warmbloods you see in the upper levels of dressage have been bred for centuries to create the ultimate sport horse. The dog world would do well to learn from the warmblood registries; all breeding animals must be inspected and evaluated. As a result, the "breeds" (they aren't technically breeds) are physically and mentally very suited for this type of work, much as thoroughbreds are genetically programmed as the ultimate running machine. If they are trained correctly, most love it. There is very little you could force a horse the size of that one to do if she wasn't inclined to do it
Of course, just like with dogs, the training can be done incorrectly and it is nothing but work for the horse. My own horse has a less than stellar work ethic, but there are parts of the dressage work that are so easy and fun for him that he clearly gets a lot of joy doing them. It gives him a huge confidence boost, and he is obviously quite proud of himself.