The ability of any two people to fall in love when there is no one else available.
Also the ability of the only person in a group of a certain gender to become the object of desire for the rest.
See also: proximity infatuation
B: "Completely Blue Lagoon Syndrome. We were stranded in an elevator together over Christmas Break, so it was inevitable."
A: "I don't get it, everybody at work thinks Suzi is such a babe!"
B: "Dude, Suzi is the *only* female in your office, of course they think she's hot, it's Blue Lagoon Syndrome!"