People all over the world learn to mount horses on the left or “near” side when they are learning to ride horses, and many of them never stop to wonder about why this is. For those of us who are curious about practices which do not seem to have a rational basis on the surface, the answer to why people mount horses from the left is fortunately very easy to find. There are two reasons why people mount horses from the left: tradition and training.
Traditionally, many people who rode horses also carried weapons and tools, since horses were used as working animals. Swords tended to be worn on the left side, as did other weapons, so people mounted their horses from the left to ensure that their swords did not bang into the horse while they were mounting. This was especially critical in the days before stirrups, when the rider had to basically vault into the saddle. If people mounted on the off side, their swords would have dragged across the back of the horse while the left leg was being swung into place, upsetting the horse and potentially causing it to bolt.
Because people mounted from the near side, they also got into the habit of gearing everything for near side handling. Saddles typically buckle on the left, as do bridles and other pieces of tack, for example, and horses were trained to be mounted and handled from the left. Although people no longer wear swords to ride, the tradition of mounting from the left has endured because it is simply what people are used to, and because tack and training are geared for near side mounting.
This brings us to the second issue, which has to do with how horses are trained. Horses do not automatically accept riders. They must be taught to wear tack and deal with riders, and in the process, they are desensitized to humans and shown that nothing terrible happens when an experienced rider is aboard. Because horses were originally prey animals, they tend to be extremely nervous, and a big part of their training is learning to stand quietly while someone approaches from the left and mounts, often unseen by the horse.
Because people mount horses on the left during training and teach them to accept near side handling, they can be shy about off side handling, because they are not accustomed to it. As a result, people tend to mount horses on the near side because this is what the animals are familiar with.
However, some trainers and riders recommend teaching a horse to accept both near and off side handling, and to mount from both sides. This makes riders and horses more flexible, and some riders believe that it balances the rider and horse by requiring them to get used to mounting either way. Famous military leaders also historically recommended teaching horses to accept off side handling, so that riders could leap onto a horse from the right in the heat of battle.