The allure of a taken man: getting stuck on a guy who's spoken for can be frustrating. Learn why you lust for a lassoed dude and how to redirect your desire.(LOVE AND LUST)

* At some point, almost every girl on the planet secretly gets hung up on a guy who's in a relationship. And no doubt, each one asks herself why she's gone gaga for a coupled-up man, knowing it's an exasperatingly futile fantasy.

"Virtually all women fall for a claimed man somewhere along the line because our brains are wired to covet things that are hard to get," says Thomas Lewis, MD, coauthor of A General Theory of Love. And while this crush is generally harmless (if you don't act on your desire), it can disrupt your emotional well-being a bit. Cosmo gets to the root of unavailable guys' appeal and how to deal with your inconvenient interest.

The Chase Is a Thrill

Ever wanted a sold-out handbag so bad, you drove to a mall three towns away to track it down? That same desperate strain of desire is what afflicts you when a guy with a GF catches your eye. "It's called frustration attraction: The more difficult something is to attain, the more we long for it," says Helen Fisher, PhD, author of Why We Love. "And the fact that he's forbidden only amplifies lust."

Here's how it works: Every time you have a craving, your brain releases dopamine into your body, which invigorates you to go after the goal. When a barrier (his having a girlfriend) prevents you from getting the target, your brain pumps out extra dopamine, making you superfocused on the hunt.

To tame your flame, "remind yourself that it may be just the challenge that's firing you up rather than the actual guy," says Arthur Aron, PhD, professor of ...