If you put aside the inevitable awkwardness of the situation, doggy humping is perfectly normal. Yes, it is a bit surprising when a young pup does it, but this behavior is actually quite common and it is usually caused by their newly emerging hormonal feelings.
Hormonal Urges
Dogs usually reach reproductive maturity very quickly, often well before their 12 months mark.
By humping on things and individuals, your pooch is training himself for what is yet to come, even though he’s pretty much oblivious of what he is actually doing.
Anxiety
A puppy may also hump random objects or people due to nervousness, apprehension and stress.
If you just brought the pup home from an animal rescue organization, he is most likely trying to deal with this newfound fear and anxiety. Give him time. He’ll adapt to the new environment quickly enough.
Displacement Behavior
According to veterinarian Laurie Bergman in her Clinician’s Brief article “this behavior describes a doggie action that arises from a blend of confusing emotions.”
If your puppy feels happy and bewildered all at the same time, he may start humping on things as a way of letting go of the pressure.