"Writing is like prostitution. First you do it for love, then a few friends. In the end, you do it for money."

It's not well known that love is an allergic reaction to a specific type of pollen. You see, there is a small uninhabited island in the South Pacific where a very rare plant, know by a select group of botanists as Mandragora aphroditis, grows.
The reproductive cycles of these plants is extremely peculiar. When the plants reach maturity, their seed pods burst, spreading millions of microscopic grains of pollen into the air. This pollen is then distributed throughout the world by the Southeast Trade winds (it's not a coincidence that cruise ships follow the Gulf Stream; they are taking advantage of the elevated aphroditis pollen levels). Eventually, eve corner of the globe is saturated with the pollen (with the exception of the Sargasso Sea. the most logical and sensible people in the world can be found living on an immense raft community at the center of the Sargasso Sea).
The pollen imbeds itself in the avioli of the lungs. Through an interaction with the infected individual's antibodies, the allergic reaction of "love" is experienced. Once bound to an antibody, the pollen become mature. the gestation period necessary for this pollen depends on the individual infected: if the person has a short attention span, the maturation can occur at an astonishing rate, but if the person is subject to co-dependence, the maturation may never occur. In fact, much of the pollen will wither and die. Even when the pollen has nearly disappeared, the initial contaminant can remain; this is what causes listless love (usually exhibited in married couples).
When the pollen has matured, it is released from it's host by the release of a toxin, which instantly allows the body to reject it. A side effect of this toxin is displayed through resentment and disillusionment. Once the pollen is released through the sweat glands, it becomes airborne, and begins its long trek home. Botanists are still unsure how it manages to incredible journey back to the island, or why it only germinates there, but they think it must have something to do with penguins and statues.
So, in short, all the love has gone to a small island in the South Pacific.