How many of us are not convinced that the love is just a feeling or a just one of the character born out of natural selection. I think it is difficult disagree with scientific approach, weather we like or not. Never mind, appearance of character by chance by natural selection is not very simple. One may just brand it, just a error of copying (mutation), filtered by natural selection and propagated by survival advantage, by it self is big thing.
Guess :- ever since mammals got separated from reptiles 350 millions years back, with appearance of sry gene, it became mandatory for sexual reproduction and also care of young ones, which could be initial expression of love and most important part altruism, which is also part love. I think these character i.e. sexuality, care of young ones, altruism are easily seen biosphere, when these are clubbed and experienced, what we get is so called divine love.