When you are mounting your horse, you create a certain amount of energy in order for your body weight to step up against gravity. This energy pushes down on the stirrup and the force pulls the saddle toward the rider. The twisting action of the saddle being pulled down toward the rider becomes increasingly uncomfortable should the rider be heavy or tall. For heavy or tall riders, more energy is needed to lift yourself, thus creating more twisting action on the saddle. In addition, if you are mounting from the ground, up to double the energy is needed.
If you take a look at the way a horse’s body is designed, you will see that it is much, much easier to tip a horse over sideways, than to roll him end-over-end. You’ll also notice that a horse’s balance is much better when he is in motion (similar to a motorcyclist). Mounting from the horse’s side while the horse is standing still makes it makes the mounting process more difficult and more uncomfortable for the horse than other mounting practices.
However, this type of mounting is traditional, and easier for the rider.
When pressure is placed in the stirrup, the horse’s weight naturally shifts towards the rider. When the rider stands farther from the horse’s body while mounting, it exerts much more twisting pressure on the horse’s back. This places more weight on the horse’s left side, than on his right, unbalancing him and making him uncomfortable. Most riders gather their reins to stop the horse from walking forward or from extending and/or swinging his neck. These natural balancing acts are what helps a horse tolerate the mounting process. To complicate the process more, when the rider swings his/her leg over the horse’s back, it shifts the horse’s weight to the right to counteract the energy of your body weight swinging over the horse’s back.
As you can see here, it not natural at all for the horse to want to stand still for the mounting process. Horses that stand still for mounting, are either trained extremely well in that area to tolerate the discomfort, or they have a rider who understands the physics of mounting and uses that information to mount her horse in the most comfortable and balanced way possible.
To learn what you can do to make the mounting process easier on your horse, view this article: Improve Your Mounting
To learn how to find the source your horse’s mounting misbehaviors, view this article: Misbehaviors While Mounting