The upside of jealousy: How does this seemingly unpleasant emotion benefit your twosome? Like so.
You Communicate Your Commitment
If your face gets flushed when he flirts, it might be a signal that you're ready to take the relationship to the next level, says Laura Guerrero, a professor of communication at Arizona State University. (Plus, what guy isn't flattered to know you want the other chick to keep her paws off?)
You Glimpse His True Colors
When you're jealous, does he offer a sincere "So sorry if that made you uncomfortable" or a stony "You're being ridiculous"? The latter might be a sign he's not the supportive guy you thought (and hoped) he was.
MORE: How to Tell if You’re in an Equal Relationship
You Stoke the Fire
For long-haul couples, realizing you want to defend that relationship (via feeling jealous) can put passion back on the front burner, which translates to hotter sex along the way. "People need to be reminded of what it's like to see a partner through the eyes of romance and possession," says Heidi Reeder,  an associate professor of communication at Boise State University. Hot, indeed.
The Science of Seeing Green
When it comes to the J-word, there are two leading theories on why you feel the way you do
The arguments:
Your gender decides
Consider it the classic Mars/Venus effect: In a 2011 study published in Personality and Individual Differences, men were more likely to interrogate their partners about the sexual nature of an affair, whereas women were more likely to be concerned with whether their mates were in love with the other woman.
MORE: Who Cheats More, Men or Women?
It depends on your bonds
Men and women with "avoidant-dismissive" attachment styles—like Mad Men's Don Draper or Summer from (500) Days of Summer—care more about sexual infidelity, while securely attached types are more distraught over emotional bond breaking, according to a Penn State study.
The Jealousy Sweet Spot
Othello had it all wrong. Giving and receiving small doses of jealousy will garner the biggest payoff. But remember: Whether your relationship blooms or goes bust hinges on how you communicate those feelings. Hand these guidelines to your inner green-eyed monster.
If You're Jealous . . .
Stay cool
Trust your gut about what feels off to you—but use your noodle when reacting, and don't jump to conclusions too quickly. Instead of biting his head off 30 seconds after something upsets you, approach him when you're cool and calm—he'll be much more likely to empathize with your concerns and find ways to reassure you, says Dr. Amir Levine, co-author of Attached.
Know your Achilles heel
Be upfront about what makes your blood boil and you'll have fewer heat-of-the-moment arguments. Managing your own triggers is clutch too: If Facebook pics of him and his ex set you off, de-friend or block whomever's giving you access to them. (A study in CyberPsychology & Behavior found that Facebook can fuel jealousy into a mounting—and addictive—cycle!)
If He's Jealous . . .
Save face for him 
If an old flame just got hired at your company and your man is sweating it, make it clear the other dude's got nothing on him with a remark like "I couldn't keep myself awake during our dates" or "He had the worst teeth," says Reeder. Your words will stick, and possibly make him laugh, the next time he's feeling insecure.
MORE: How Jealousy Can Be a Good Thing
Don't fight it
When he catches you eyeing a cute guy on the street, don't tell him you were staring at a bed of flowers. (We don't fall for it, and neither do they.) Acknowledge the glance and defuse the situation by saying something like "He looks just like a guy I went to school with" or "I really like that jacket he has on," says Reeder.
Manage his way 
Few dudes want to hash out the particulars of their jealous feelings. Instead of unpacking his emotions, try talking in terms of solutions and problem-solving, says Reeder. If he's feeling green, ask him, "What would make you feel more comfortable?" or "Would it help if I did this?"