What makes riding so interesting and addictive is that it is a lot of things. It is a craft. It is an art. It is a sport. It is also a science. In some ways it is simply applied physics. It has parallels with the practice of medicine. It is a healing art in the sense of physical therapy. It is applied psychology. It also has an intuitive, psychic side that must not be underestimated. It has things in common with yoga, pilates, dance, and music. These different aspects draw different types of people, and in order to go to the top and fulfill one's potential, the rider has to try and become as competent as possible in all these areas. Nobody can possibly master them all, which is why especially the truly great riders always emphasize that it takes more than one lifetime to master dressage.
Focusing on one aspect to the exclusion of the others lets the rider fall short of his or her potential. Somebody who sees riding exclusively as a sport and wants nothing to do with the other aspects will always remain on the surface. Somebody who sees it only as an art and does not take the technical, craftsmanship side or the athletic side seriously, will be held back by these shortcomings. Someone who gets too wrapped up in the physics and technique and never develops feel and intuition, will not get very far, either, etc.
The best riders I have met all combined a fairly high competence level in most of these areas. They were all fit and athletic, highly intuitive, with an excellent understanding of psychology, biomechanics and conformation, and they had spent their entire lifetime practicing the craftsmanship side until it had become second nature, so that they were able to transcend technique and leave the text book path behind when necessary in order to find a practical solution to a problem through the application of intuition and artistic creativity.