Why does my male dog hump my other male dog?

Humping can be done for many different reasons, could be for dominance, can be a play behavior gone a little strange, some do it out of boredom or even simply for sexual pleasure and some to help with anxiety.

If he is not compulsive about it, if it only happens here and there or with certain type of dogs and can be stopped easily, I wouldnt worry about it. Its when the behavior becomes compulsive, happens all the time or is not easy to stop that I start to ask myself more questions. Look at the rest of the dog's behavior, other ways he expresses his feelings, is the dog well stimulated both physically and mentally, is the dog dominant in other social interactions with dogs, is the dog fixed, that sort of thing.

This action is to establish dominance. It is also a form of play to some dogs, but not all. One dog may find it playful and another may not appreciate being "dominated" if it is not a submissive dog itself.

Most male-male companions or even female-female and even opposite sexes will hump one another for dominance. This could be maybe one of them isn't fixed or just one of them trying to establish dominance over the other.