The Root of the Behavior
Dogs mount other dogs for many different reasons. This behavior is found in both male and female dogs. Sexual desire can be one reason dogs chooses to engage in mounting behavior. However, canines are a species that mainly desire sexual activity for reproductive purposes. Because of this, an intact female dog is uninterested in amorous male attention except for those two times a year when she is at her peak ovulation point. It is a short window of time lasting only a few days. During this period of time known as "standing heat," she will overly flirt with a male and will even attempt to mount him or other canine members of the household.
Male dogs possess the ability to procreate at any time. Their strong desire for reproduction remains consistent. Dogs possess a powerful sense of smell, and experienced studs can detect when a female dog is fertile. Many will not mount a female until the right time to help ensure a viable pregnancy. It is quite common for an experienced male stud dog with a scent of a female in heat to begin mounting other female dogs or even males for "release" from his frustration.
But mounting is not strictly limited to intact dogs. Spayed and neutered dogs will also hump other dogs or inanimate objects. Though this could be sexual in nature, it is difficult to determine whether or not that is the key motivator for the action.
It is quite common for many puppies and even adult dogs to mount one another in play. Dogs at play can easily become overstimulated. Since dogs do not know how to cope with their excess energy and frustration level when overly aroused by play, it can lead to mounting behavior.
Stress and change of routine bring out some unusual behaviors in our dogs. Because dogs are not fans of change, they can find any interruption in their lives to be upsetting. Humping other dogs, and sometimes inanimate objects, is one detectable way dogs deal with the stresses they experience as a result of unusual or unexpected activity in their homes.
More rarely, humping can be indicative of a medical problem or dogs trying to assert a social hierarchy in a home. Dogs that feel uncertain of where they fit in the "pecking order" will occasionally try to mount another dog to gauge a reaction. From the response they receive, they are more easily able to determine where they fit in the household. This can also work in reverse with the lead dog in a home teaching subordinates their place.
Encouraging the Behavior
Care must be taken to strictly observe all play time and monitor any mounting behavior. Many dogs take exception to being humped and view it as highly offensive. At the first signs of excessive arousal due to roughhousing, it is best to redirect or stop play to prevent escalated actions which could potentially lead to a dogfight.
Know your dog well to help you understand the motivation behind mounting. Has there been any change in your house recently? Have you just returned from a weekend getaway? Did your dog have an overnight in a kennel? Is there a new baby in your home? Did you buy a new couch or move an old one to a new location? All of these can, and do, affect dogs and can result in stress-induced mounting.
Should your dog be engaging in mounting and you suspect it is related to change, extra reassurance from you will go a long way to help the situation. When you find your dog mounting another dog or an object, simply remove the dog from the situation and provide some additional comfort on a one on one basis. This can be done through a bit of extra snuggle time on the couch together, a walk, or even a little game of frisbee in your yard. All of these familiar and beloved activities help provide comfort to the stressed doggy brain. Refocusing on your routine will help reassure Fido that all is well.
Other Solutions and Considerations
Regardless of what is the cause, mounting is generally a harmless dog expression of emotion. While care should be taken to monitor what is eliciting the behavior, it can mostly be easily managed by simply redirecting the dog to a different activity except in the case of medical issues or compulsive behavior in which cases veterinary and or behavior modification training should be immediately sought.