You got your pet desexed because you are an awesome, dedicated, responsible owner. Or perhaps you adopted a pet from the pound and it came desexed with proof of papers, so why does your pet insist on humping? Shouldn't that behaviour be gone along with the testicles and uterus? How is it that a pet can still display sexual signs even after they have been desexed? The answer is quite simple.
You see, even though you have removed some of the testosterone, you haven't removed ALL of the testosterone and the sex hormone can still remain in your pets system. As a result, it seems every dog is humping around, although less frequently.
If you see you pet mounting something, anything or cough cough anyone it is either a sign of hormonal driven behaviour or displacement behaviour.
It has been thought of previously - dogs that mount their owners leg are showing signs of dominance, but studies have shown this is not the case. When a dog behaves in such a manner, it is displaying signs of excitement and anxiety both simultaneously. This can happen when the dog gets excited about seeing someone, or going somewhere and also knocks over the vase in the mayhem, causing the owner to yell. The dog is expressing two emotions at once - excitement about going somewhere and anxiety with the owner getting mad and as a result, the dog may start humping as he is feeling conflicted.
Sometimes, dogs hump just to get attention. When dogs hump, the owners (or victims) voice raises an octave. There could be shouting and squealing which sounds very exciting to a dog. This is learnt behaviour. The dog gets a reaction to what it is doing, so will continue to do while there is a reaction. Makes sense huh.
So how do you stop that? Just ignore the dog. Remove yourself from the scene and sooner or later your friend will get bored as there is no exciting reaction happening to his trick that he has mastered.
Sometimes, the humping is due to a UTI or a hormonal disease that can mimic an increase of testosterone. If the dog is suddenly mounting, it is advised that you seek veterinary attention to rule out any infection or disease.
Mounting doesn't bother the dog, it bothers the owner. Owners view mounting as a sexual act, however dogs don't see it that way.