Stan, I'll correct myself - I said that it is all about dominance, which is not always the case. However, it is only sexual if it is done by a bitch in heat or the males around a bitch in heat. Dogs do not have sexual desires like humans do - their sexual instincts are purely for procreation. When the bitch is in heat, it's time to mate. Simple as that. They do not fall in love, nor do they enjoy sex for fun.
When it comes to mounting, there are various reasons besides sex during a heat cycle. These can be medical such as hormonal imbalances, obsessive compulsion, anxiety, or even a skin irritation that simply needs to be scratched. In puppies, mounting can be a form of play. For the most part, however, if all medical reasons are ruled out, the reason is mostly the establishment of dominance.
Dogs are pack animals period. Just because they live with humans, doesn't mean they aren't part of a pack. When humans own a dog, the human-dog family become a pack. Some dogs are bred to be independent - such as livestock guardian dogs who need to be on their own to protect the cattle, sheep, or other livestock. Otherwise, they are strongly pack-oriented. They face separation anxiety issues when left home alone or tied up outside their entire lives. Having a dog in your home makes your family a pack and therefore hiearchy must be established.
Pillows, teddy bears, and other belongings are an extension of the family member. When the dog is unsure of his position in the pack, he may attempt to establish dominance in a subtle way -- by mounting your belongings with or without your presence. Or if he is a very dominant dog, he will establish his position in agressive ways - mounting YOU, nipping, barking at you, etc.
When mounting is occasional and done during play, then it may simply be an act of excitement. However, if you are in another room and walk in on your dog humping your pillow, he is probably not sure of his position in the pack.
The only time it is sexual is if it is a female in heat or a male around a female in heat -- even if that bitch is two blocks away.
My American Eskimo is always trying to win dominance over me. My husband is clearly alpha-male, I am alpha-female, and my Eskie claims beta-male (even if she is female) and tries to win alpha-female. She does this by pressing me, trying to see what she can get away with. I often have to reestablish alpha-female and remind her that she isn't it. She backs down, but will occasionally hump my pillow when she thinks I'm not looking. If we dogsit my brother's goofy lab, she establishes dominance over him and will mount him even if he's larger than her. Lucky for her, he submits easily and acknowledges her as dominant over him. She never mounts during playtime with me or my husband.
Here are some links that discuss mounting and mounting for dominance:
http://www.canineconcepts.co.uk/ccp51/cc/dog-behaviour/mounting-dog.shtmlhttp://www.veterinarypartner.com/Content.plx?P=A&A=1498http://www.freedogtraininginfo.com/131/dog-mounting-humping-offensive-sniffing/http://www.dogsobediencetraining.com/dog_training/behavior/stop_dog_mounting_behavior.htmBTW, I'm so sorry for hijacking this thread!