I have yet to see Love proved to be an emotion.
But all this is inconsequential until we hammer out what love is anyway below is the crux of the arguement.
That is a good question actually, but I find it hard to distinguish the difference between "mental state" and "emotion". If we go by the parameters of Intimacy, Passion, and Commitment that together create love, I would say that love would be a description of a level at which all three of those emotion reached. To put it more simply, since others have pointed out that love is relative, when Intimacy, Passion, and Commitment all reach X level, then the subject's combined emotion would be love.
I say "X level" because I do think that science can determine levels of intimacy, passion, and commitment, based on personal actions/reactions, observable brain scans, and yes, even psychological studies involving a questionaire in which there is a large enough sample base to produce reliable statistics.
If level x is something that is achieved after such a combination then it is not truly an emotion, by trying to say love is merely an emotion measurable in physical repsonse you are characterising love merely as a robust concern. x loves y then x wants to benefit and be with y he has wants and love is a way of satisfying these wants for emotional gratification. Using the above definition however we can see that love is a culmination of various factors not merely desires or emotional impulses. So it favours the scruton and fisher definition of love, the idea that the union is one of concern, that forms over a given time period. That love only transpires once a relationship has formed not merely our responses or actions towards one another.
The emotion view of love, one that can be measured is a shallow view of love. To feel an emotion about an object you evaluate it and respond to it, if you are angry at something you see it as offensive if you fear something you see it as dangerous. The problem with this concept of love is, it is an evaluative response and fails to take into account the changing aspects of love or the compex facets that cause love even when the evaluative process deems that person unworthy. As a logical analysis it does not distinguish love from respect or admiration so therefore leads us on to what we have talked about which is the blending of emotions a very complex idea (1. a blend of emotions which I think we both agree on) that intimates a interpersonal relationship the idea that love (again) is a relationship where your emotions are deeply dependant on intimacy and co-dependance (commitment see above definition, commitment is interpersonal not one wa).
This suggests that love is not a prescence or state that we can point to at any one time but rather a derivative of the history of the relationship, the love is created by the emotional dependance of the simultaneous evaluative state of both parties. The emotional interdependance that results from this can explain the depth of love that allows old fogeys to love each other despite certain criteria of their evaluation (ie. looks) failing because there are other factors and history.
What questions you have to ask is, why if love is an emotion (an evaluative response) or a complex emotion (multiple combined emotions) what makes the fact that you have loved the person in the past continue into the future. Why would you not find someone with the same or better values and transfer that emotion to someone else. The key seems to be interpersonal relationships.
I have drawn of two philosphical ideas of love there, the unionist and the emotionally complex form of love. I don't believe in distinguishing between the two from personal experience as I believe (a duelistic view) true love involves not only emotional interdependance and the evaluative process but also the union of two peoples concerns that makes true love, that you must care about the other persons pain, ambitions and happiness. Take pleasure in their pleasure and strive for it but also gain your own needs as well (so its a tertiary view really love as a concern).
How do you measure these factors in a scientific manner