What is love? Tough question. So, an easier question: what are the physical aspects of love?
When you love someone or something, your brain uses a portion of the energy available to your body to (at the very least) remember it. Of the billions of bits of information available to be learned and remembered, nerve cells are set aside and energy consumed as your brain sets up a neural pattern that will recognize the one you love, and incorporates your love of this person into your physical being.
Having your neural network slowly altered, your thought patterns will flow along these new neural pathways (just as they do with all the other neural pathways). As a result, some thoughts will dwell on the one you love. More energy and time is spent focusing on your loved one; in a sense, you have given a small bit of your life in reverence to the one you love.
Thinking often begets action, and always colors it. Loving someone thus affects the energy used by your entire physical being (which furthermore affects your life). Because your actions are affected, your immediate surroundings are affected as well. As everything ultimately affects everything else (even if it is solely through increasing the entropy in the universe), your love will nudge the balance of the universe.
All this, just because you love someone.
So, what is the physical aspect of love? It is burning energy to reinforce a relationship to something outside of your physical being. It is incorporating that of another into yourself, and projecting that back outward from yourself.