He’s a lot like her
You choose your relationships based on compatibility, so it shouldn’t be surprising to find out that your best friend and your girlfriend are shockingly similar. Your best friend is likely to have some of the same qualities and behavior patterns as your girlfriend, For example, you might be extroverted, but feel more comfortable around friends who are introverted. If your girlfriend is also more of an introvert, she’ll understand and relate to your best friend on a different level. But that doesn’t mean she’s about to jump ship—remember, she’s with you because you complement those qualities in a way he can’t.

It’s easier for her to relax around him
When she’s with you, she’s on her best behavior (most of the time), because she wants you to see her as a perfect, sexy, awesome girlfriend. But when she’s around your best friend, the pressure’s off. “Face it—relationships aren’t easy. They’re risky. They’re challenging. And they’re not all good times. If she’s into your best friend, it’s because she’s not dating him—she’s merely attracted. It’s a lot easier to feel attraction, flirt, and consider the possibilities than it is to dive in and actually be with someone for real.”Making her see that he’s not all he’s cracked up to be means taking a chance. Let him spend time with her, and you’ll see if, and how much, she misses you. He’s the object of her attraction because he feels safe, but if she’s forced to hang out with him because you’re late to dinner, she’ll realize he’s not you!” 

She’s trying to impress you
She’s smart—she knows that if she wins over your friends, they’ll be her champions forever (especially in risky situations, like when you’re drunk at a bachelor party). Plus, she knows that you don’t want to hear her bashing your lifelong basketball buddy. She knows that one way to get closer to you is to acknowledge that you have great taste in friends. Seriously, would you be happier if she repeatedly told you how much she hated your best friend? Because he is your best friend, it’s possible that you’re playing him up a lot—especially if you have a long history together. You may not realize that you’re creating an award-winning ad campaign for him simply by singing his praises and including him whenever possible. Start opting out of his invitations every now and then, bringing other friends around, and sing the praises of others in addition to him. 

He’s mysterious
Models, movie stars, and public figures are super attractive because you’re only given a shallow image of them to covet. Chances are, she thinks your best friend is pretty darn perfect, because she’s never seen him at his worst. And, well, let’s just say she probably doesn’t always see you at your best. So what she sees is a guy who’s a lot like her awesome boyfriend, but without all of her boyfriend’s flaws. Oops.This one’s easy to fix: Expose him for who he really is. Take her over to his apartment once in a while, so she can see the stacks of dirty dishes and the refrigerator full of beer and protein bars. Give her an accurate description of him—tell her a funny story or two from your past—so she’s not only hearing about what a great guy he is. Just make sure you’re exposing his real-person-ness, not divulging his dirty secrets or openly bashing him. You want her to see him as a regular guy, not a loser.

She’s trying to make you jealous
Some people think a little jealousy will keep a guy on his toes, and she may be exaggerating her attraction to him to keep you working to win her over. Because she doesn’t actually care about whether he’s attracted to her, it’s much easier for her to flirt and engage with him. Trying to make you jealous isn’t a deal-breaker, but that doesn’t mean you should engage. The best thing you can do is to not become jealous and possessive. You can all enjoy doing some things together. But if she gives you reason to worry that she likes him as more than a friend, try arranging a date for him so you can double and nip those feelings in the bud.