I'm going to guess that the "self-evident reason" is that some higher god created man and therefore man is OBVIOUSLY superior, since dogs were placed on earth merely to be our servants.
C'mon, it's so SELF-EVIDENT.
Indeed.  Next.
		
		
	 
I think it's simpler than that.  There is a moral code that says that murder is the worst thing you can do.  There are numerous reasons for this, from genetic, to social, to religious, to self-serving.  The definition of murder is The choice to save the dog is not murder.  But it is tantamount to murder because you have a living human's life in your hands, and for whatever reason, you decide to let him die.
Whether you consider it negligence, caniphilia, loyalty, or something else, the decision to save the dog over the human is a decision to make a human die.
If you told me that you really believed you could possibly save both, but were certain the dog was going to die if you didn't try to save it first, then it might be different.  But in this case, saving the drowning human is the only morally correct answer.