That is a fantastic question, one that many probably struggle with a bit. I think Tranquility had it pretty close to the way I see it. Also, I think we base what we know as love off of what we were shown in the past as we were developing emotionally. To me, I believe love is compassion, it is also understanding. A good relationship requires trust, but you don't need a relationship to love someone.
Love is an outward expression of how we feel about ourselves and it requires a whole lot of unselfish thinking and actions.