Farva (Ben):
This actually deals with my area of study. Not to say that means anything I say makes any sense at all but I will attempt to shed some light on your questions.
What is "love" here?
Well, technically "love" isn't really anything. What happens, both physically and mentally, when we love greatly depends on the society in which we live. However, we can measure the neurological response to the feeling associated with the situations in which "love" is commonly used to describe. Affection, commitment, intimacy, passion, et cetera.
"Love" is just a term that describes a feeling and the resulting behavior.
Everybody here is answering your question about "love" the EMOTION. I don't think your wife is asking about that at all. She's asking about love the FEELING.
It's all the same actually. At the primitive level we're talking about reproduction and biologically implanted family bonds, just like hunger. Most of the behavior associated behavior of love has nothing to do with neuroscience and everything to do with culture. Certainly love is expressed differently around the world.
So back to the beginning, what we measure is the reaction of the brain in "love" situations. This proves that this activity is happening and that the subject does have an observable and verifiably different reaction to people he/she loves to those he/she does not.
I would consider this to be proof for the original poster's wife.