All animals want to survive. With this in mind, lets talk about some natural drugs that your body creates to create this feeling of "love".
P.E.A. is a natural anti-depressant that the body makes, kind of like dopamine. This is the most affiliated with the nickname "love drug". It creates the need to reproduce sexually. This is the primal instinct; to reproduce and pass on your genetic material.
Now, as you stay with one partner, you may find that the feeling changes a bit; this is P.E.A. turning into oxytocin. This is a deeper kind of love that humans develop once they have been with a partner for long enough. High levels of oxytocin are found when a mother has a child, usually found during breastfeeding.
There are many natural drugs that your body produces to create the feeling of love. If you have any questions, let me know! Hope this helps!