One behavior I often observe with my dogs is that they'll put their heads on or above the backs of other dogs.
Why do they do this?
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This is an attempt to assert dominance over other dogs.
In the dog world, simple behaviors are used to display dominance over each other. For instance, have you ever seen two dogs stare at each other until one looks away? They are intimidating one another to establish a pecking-order.
Other aggressive/dominant behaviors that dogs display (and you can watch out for them)
There are many others, see this link for more details
Dogs are social animals and have a large repertoire of communicative signals and behaviors. Additionally the interactions between two dogs also involve many "learned" kind of interactions. One dog can learn that the other dog will let him do something. If that something is associated to a context that both dogs appreciate, it can become a learned habit. For example your dog could do that as a way to relax and send a "I'm calm and relaxed" signal to the other dog.
Observe when your dog is doing that and then you tell us what you think are his reasons.
Referring to @Chris's answer involving dominance.
The outdated "dominance theory" has been debunked in every aspects, over the last ten years and by many authors (including Bradshaw, Dunbar, Eaton and many others).
In short here are some key arguments:
The dominance theory is based on the observation of captive wolves packs, where the individuals are constrained to stay in the pack and are, in most cases, unrelated
Observations of wild wolves packs is quite different in term of social structure and hierarchy. A typical wolf pack involve a single breeding pair, along with the previous years litters and the cubs. In that sense, the alpha pair are simply the parents.
Dogs do not socially behave like wolves (captive or wild). Studies and observations of feral dogs (eg. Pariah dogs in India) revealed that the "pack" are very different from wolves packs. There is no single breeding pair, the individual from different groups often interact without fighting (which is very different from the behaviour of wolves from different packs: they avoid each other but will fight in almost all cases if they do meet).
The "dominance theory" took all of that a step further by claiming that dogs will keep that social structure even in their interactions with humans. No scientific data supports that assertion.
Dominance in Dogs: Fact or Fiction? (Eaton - book)
Dominance in domestic dogs-useful construct or bad habit? (Bradshaw - paper - see references inside)
The other answer regarding dominance is one reason. However many dogs do this as a play behavior as well. While playing more dominant dogs will often switch roles and act submissive towards other dogs. We can guess that they do this to encourage the less dominant dog to play and that they won't get in "trouble" for it.
If the dog is being forceful or is not correctly reading signs that the other dogs dislikes the behavior than I would step in and not allow it. Otherwise it is part of normal play.