Just like every other instinct. It is driven by chemicals and is the result of chemicals. After all, humans are just a self-replicating chemical reaction, just like every other life form on this planet. So unless there is magic involved then love must rise out of the material world and must be driven by chemicals. Therefore it is an instinct. I am very much interested to see the reference of this "scientifically" proven theory, and I am much inclined to believe otherwise to your definition of science is very loosely based.
It is scientifically proven that love does not exist and it is where two people's immune systems "magnetically" connect which sums up attraction. Being able to like the person is different situation because that is just your mind like you do with friends. If love was real then what are the chances of so many couples actually being in love out of the millions of people out there in the world so basically it means that love is an option and you can connect with anyone that you set your mind to.