A debilitating disease that causes one to believe that a specific girl (or boy, according to taste) is special and unique, and that it is very, very important that one not do anything wrong to spoil one's chance of snagging this very special creature.
The irony is that this condition is actually repulsive to the opposite sex since those afflicted typically act shy, nervous and excessively ingratiating toward those they would impress. The easiest way to scare away a potential seductee is to want to seduce her (or him), which commonly leads to the sort of pathetic scene where a lovestruck fool makes a spectacle of him (or her) self chasing someone who is obviously, and often obnoxiously, not interested.
Most of us have been on both sides of this dynamic, regardless of physical attractiveness or lack thereof. Being beautiful is not necessarily effective protection since taste varies and no one is universally appealing. The only people who are immune to this disease are those narcissists who already have the affection of the only people who matter to them, namely themselves, and it is these who attract the greatest number of silly admirers.
While there may be biological and evolutionary rationales for these seemingly nonsensical patterns of behavior, I prefer to think that God is a f****** sadist.
The standard prescription for this disease is to GFTOW (PG-13 version: Go Find Ten Other Women), since many believe that experiencing a variety of other partners will cure one of the delusion that any one person is special.