Testosterone has a way of supercharging many horses' psyches. In some I call it testosterone toxicity. Otherwise gentle riding and performance stallions can get very obnoxious when presented with a mare in heat. Even a mature stallion like amateur Sue Attasani's Thoroughbred Simbalu, a perfect gentleman while competing in regular working hunter divisions at shows, becomes quite a handful in the breeding shed. In natural herds young colts quickly learn horse-breeding etiquette.
Older studs rule the herd through domination.
A colt learns to approach mares with caution, carefully reading the mare's receptivity by observing her responses to his advances before initiating copulation. An experienced mare's kicks would slow and correct an overly amorous young stallion. I've found that stallions kept in either a natural herd or with geldings are more docile and easier to deal with during breeding than isolated stallions. One client keeps 10 stallions in one 200-acre pasture. Each stallion maintains a harem of about five mares, and, while there is some fighting between the stallions, injuries have been minor.