Why is the love I feel for my two daughters far stronger than any love I've felt for anybody else?

I suppose the biologically hard-headed answer is that creatures who have an inclination to care more about their own offspring than anyone else tend to do better at spreading their own genes. Fortunately, that natural selection explanation does not make the love you feel any less real. It means you didn't choose to love your daughters so much, but whom ever thought you did?