Let's be clear about this. The reason to test things on animals is so that you are not testing them on humans. Humans have more value than animals. That's just a basic fact of life for, I think, the majority of the human population. If a living creature must take risks, better it be a rat than a person.
If you disagree that humans are more valuable than animals, I simply don't understand how you think. If you had a choice to save the life of a puppy or a baby, and you aren't sure which you would choose, I would say your way of thinking is more monstrous than research done on rodents.
If you would choose the human baby without hesitation, then you do place more value on human life than animals, and the argument is just one of degree.
And how do you arrive at that conclusion?
There are two ways I can take your question, I'll try to answer both.
I conclude that most people place human life above animal life based on personal experience. I've rarely met anyone who was as upset about animal death as human death; I don't know that I've ever met someone who claimed to put the life of an animal on the same level as a human child; you see people willing to do extraordinary things to save other people far, far more often than animals. So that's where I get the conclusion that most people value humans over animals.
As to where I personally get that value from, it's mostly an issue of intelligence. That's a very general explanation, but the ability to communicate is a big factor, perhaps because we are such social creatures. There's also the ability to understand concepts like death, the ability to feel and express pain, etc. There's even the whole idea of 'sticking with your own kind' to some extent.
Now, there may be individual cases where my values differ. I may care about a pet more than a person I hate. But in general, I consider humans worth more than other life. It is my belief that everyone makes their own judgements about the values of various types of life, and no one truly considers all life of equal value (unless, perhaps, that value is zero).