I don't think that it is necessary to motivate your choice from 'how important' human lives are. But from the fact that humans are responsible for other humans, in a different way from the way they are responsible for other animals.
Species naturally advance their own genes. By the standards of many biological theories, that is what makes a species a species. And one of the most logical ways to advance them is to preserve them. So any non-domesticated animal would choose to save the members of its own species before those of another.
[By domestication I mean when one species -- any species (we are not the only species that domesticates others http://www.nytimes.com/2002/10/15/science/before-adam-and-eve-the-farmers-were-termites.html) -- adapts the behavior of another for its own ends.]
We are non-domesticated animals. So it is only natural for us to do the same.
Your own dog is one of the very few exceptions. Heavily domesticated species like dogs and horses have been bred and raised to value humans above themselves. The dog might save the human. Well-bred horses do quite uncomfortable or dangerous things to protect their riders or drivers.
But rather than some absolute 'value', I think this is better looked at as the contents of a certain kind of social contract. We agree to take care of our own lives, and to trust nature more with the lives of everything else.
The dog has been brought into that contract in another way -- its existence is due to our choice. The entire species would not exist, had we not adopted it as our own set of personal servants. (We have evidence that even the most peaceful, primitive, vegetarian societies did so, keeping them as sanitary agents -- garbage disposals and walking napkins). Likewise, the cows people eat would not exist if we had not fostered them, and we feel entitled to determine what purpose they should serve.
I think we should avoid a moral basis in absolute human value, which seems to me to culminate in the tradition of trophy hunting, where human comfort and enjoyment is of value, and the lives of animals hold little intrinsic value. Instead, we should look at the agreements 'negotiated' by our societies and our genes, and consider where everyone's best interests lie, but accept that we will always value those more like ourselves higher than others to some degree.