I can't seem to dig up my old research, but it was based on Sternberg's Triangular theory of love. You guys should give it go. It's not perfect, but I think the concept was explained very well.
http://www.robertjsternberg.com/love/

When you just have passion (lust), It's only an infatuation. The love most of us are going for need to contain 3 factors: Passion, Intimacy, and Commitment. Of course, there are always exception, but what is love without the 3s?