Well, drat. Me and my Hobbesian views mixed with a practical and scientific mind...
The origin of Love and what makes emotions are biochemical reactions... Surprisingly enough, a different set of reactions also move you forward, but I doubt that's the answer.
This always happens when I'm stumped by something, which is usually what happens around riddles when I haven't heard them before or know their origin and thereby can use the psychological profile of the area / time to think it out... (Yep, I'm that guy)
It is stronger than Desire, and more powerful than Want, but is not a base feeling. It is central to the soul...
Is it thought? The mind? How about Need?