Love is dopamine being released into your brain to ensure that you stay with the person you mate with long enough to raise the children to an acceptable level in which they no longer need two parents. Humans are the weakest species on the planet at infancy, and the only species that cannot survive on it's own for quite a long time. Normally 4 years, which is why that "spark" as we call it, seems to die off after about 3-4 years, even without kids. It's primal instinct to further the species.
Gotta love evolution