Its the fact that stallions have one purpose in life...to breed. That desire/instinct will precede all training if necessary. Stallions require better or specialized fencing and handling. Stallions are at best, unpredictable, at worst dangerous, when there is a mare around in season. Therefore it is often written and with good cause that stallions shouldn't be kept as riding horses. A good stallion makes an excellent gelding! Unless you are part of a breeding program, I too would recommend having your stallion gelded.