Could you imagine if people peed on each other as a means to exert dominance? Jeeez, the world is already crazy enough..
Yeah, this is the dogs way of marking territory, and exerting dominance. It's the animals way of letting other animals know:
"Hey this is mine, back off".
Dogs mark their territory by peeing. It is a way of showing other dogs that it's theirs. This is the reason why dogs are always peeing where other dogs have urinated.
I suppose the retriever is showing everyone they are his. My bull mastiff likes to urinate on our other animals in the house as well showing a dominance. It is a strange thing but i think it is the simplest explanation. Showing dominance and what belongs to them leaving their scent.