1. Head ove heels in love feeling diminises over time.There is less urgency to contstantly woo a potential partner to ensure they are yours.
Once a couple has been together for a long time and are in a committed relationship, they tend to no longer use their "dating" behavior. That is, the woman might no longer put on her makeup every day, or she'll only shave her legs once in a while. The man no longer brings her flowers or buys her cute, romantic greeting cards. They have won each other over, they no longer have to "woo" or court each other.
2. Circulating levels of NGF are elevated among subjects in love, suggesting an important role for this molecule in the social chemistry of human being.
"Social chemistry" is a sort of "spark" you feel when you've met someone that you think might be your soul mate. Your heartbeat quickens when you think of him, you get butterflies in your stomach, you even get a little giddy and silly. That feeling cannot last forever, of course, and apparently these scientists are trying to find a biological explanation for it.