Long term love is, and has been recognized as an addiction. The image of someone's face, causes a burst of seratonin in the brain, thus making their presence appealing, and eventually necessary for one's mental health.

People do have instincts, it's a known fact. Humans have inborn fear of insects, arachnids, and reptiles,at well as anything larger then us. We also seek sex, and food constantly. That's the definition of instinct, since no choice is involved, whatsoever.

And the way chemicals like PCP and LSD work in the brain is by attaching to neurotransmitter receivers, and mimicking natural chemicals which are present in your brain. They cause euphoria by mimicking chemicals which signal a seratonin release. People become delusional when they are in love. Some even to the point of being unbalanced. Stalkers in a lot of cases claim to be in love, and that the object of their affection is in love with them. Neither of these things would be true if the kind of love you claim exists, really did.