Mark Twain wrote, “There is a charm about the forbidden that makes it unspeakably desirable.”
The longing for the unattainable, the sublime misery of unquenched desire is “La Douleur Exquise”. Translated literally from the French as “the exquisite pain,” this is a pain almost like unrequited love, but heightened by the truth that the love is impossible, improbable or reckless.
Your professor. Your boss. A married man. The swarthy handyman. Johnny Depp.
It helps to remember that he or she is likely unattainable for good reasons. Fran Creffield gave some good advice in this eHarmony article, “Should you ever date your boss?”.
If the object of your desires is a married man, an article written by men in "Answers from Men", might dissuade you from the practice the writers call “one of the most empty endeavors that you could embark upon.”
Paul Bloom, professor of psychology at Yale, wrote, “The enjoyment we get from something is powerfully influenced by what we think that thing really is [emphasis mine]. This is true for intellectual pleasures ... as well for pleasures that seem simpler and more animalistic, such as the satisfaction of hunger and lust.”
In essence, because the object of our desires is unknowable — forbidden fruit — our thoughts construct a fiction, a fairy tale about who this person is. The less we truly know the person, the more favorable a fantasy our brain creates. Our loins help.
Anthropologist Helen Fisher posits that human love travels three routes through the brain. In her article, Brains Do It: Lust, Attraction and Attachment, that the three types of love mentioned in the title travel through three distinct, primordial circuits in the brain.
Passion sparks lurid visions of short term gains — lust and conquest, if you will.