Though a bit awkward, mounting behaviors are quite common in pups, whether due to playtime, hormonal feelings, or displacement behavior.
Hormonal Urges
ASPCA states that this may be a type of precursor to sexuality in canines, since dogs reach reproductive maturity quite early, sometimes well before a year old.
Via this behavior your pup is “training” himself for what is yet to come, even they are oblivious of what is actually going on.
Dogs become capable of reproducing around 6 months old, which is when the humping behavior usually begins. Puppies are actually ready to mate at that point (unless they are already fixed, that is).
Displacement Behavior
Another reason might be canine “displacement behavior.” According to Laurie Bergman, a veterinarian and author of “Clinician’s Brief,” this doggie action seems to come from a blend of confusing emotions in young pups.
Namely, they can feel happy and bewildered at the same time, and the humping might be a way of releasing the pressure.