Ok, maybe this is only obvious to me...
I can't remember who said this: "Why is it we never know when love begins, but we always know when it ends?" Love is defined by the pain you feel when the thing, the person you love is taken away from you.
You know you love someone when even the thought of them being taken away from you fills you with dread and fear, and if they are... a part of your heart, your very soul is taken away with them, and you can never quite forget the pain of the loss.
Forget about chemical reactions in the brain, or religious quotations, or whatever - in plain practical terms, to love someone or something is to be so attached to them that you are emotionally hurt beyond all measure when that person or thing is taken away.