An aggressively, excessively sexual male dog is more than just embarrassing to you and troublesome to the objects of his affection. It may be that his excessive sexual behavior like mounting is rooted in a medical issue. Have your dog checked out by a veterinarian before attempting behavioral training or more permanent solutions.
Your dog's obsessive mounting or otherwise becoming excessively friendly with dogs, legs, toys and more could be related to a medical condition. Issues like priapism, allergies, UTIs and excessive hormone production can contribute to apparently excessive sexuality. Before attempting to curb your dog's behavior with training, then, take him in for a veterinary examination to rule out treatable conditions.
A dog's extreme sexuality can be caused by behavioral issues -- ones that may or may not be sexual in nature. For example, he may mount other dogs as a form of play and not as a sexual maneuver at all. He may resort to humping as a response to exciting or stressful situations, or he may just enjoy the feeling and become obsessed. Dogs left alone too much are prone to this.
Neutering can curb your dog's sexual appetite by reducing his hormones, but it isn't necessarily a cure for the sexually aggressive canine. Neutered dogs who had been allowed to sew their oats can experience the physical pleasure of sexual behavior and may go on exhibiting it even after being fixed. This is why it's generally best to neuter dogs young, before they develop a taste for sexual behavior.
While no foolproof cure exists for extreme sexuality in a dog, consistent training can help your dog abstain from giving in to his desires. This typically consists of both distraction and negative reinforcement. For example, teach your dog a command like "Sit" or "Lie Down" using standard practices. Once he knows the command, give the command any time he begins to mount -- his desire to perform the command and receive the treat will outweigh his desire to mount. Until he learns this command, though, or any other time he mounts, give him a stern "No" command. Remove him from the object he is mounting -- even if it's you -- and put him in an isolated area for a few minutes before letting him back out. Do this consistently every single time he mounts, and eventually he will learn mounting gets a negative reaction and results in loss of attention. His distaste for the disciplinary action will teach him that mounting isn't so great after all.
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