For every 10 friends you make, you gain one enemy, author Eric Barker notes in his recently released book "Plays Well with Others: The Surprising Science Behind Why Everything You Know About Relationships Is (Mostly) Wrong."

But in the space between friends and enemies are "frenemies."

A frenemy can be a person you just don't like all that much. Some folks might even have a medical condition that makes them hard to befriend—an example Barker gives in his book is someone with narcissistic personality disorder.

A frenemy can cause you even more stress than an enemy, Barker notes.

"Why are frenemies more stressful than enemies?" he writes. "It's the unpredictability. You know what to expect from enemies and supportive friends — but with those ambivalent ones you're always on edge."

You probably have at least one person in your life, like a close family member or a co-worker, who falls into one or both of these categories. In fact, Barker writes, "ambivalent friends make up half our relationships."

"Studies find that we don't see them any less often than supportive friends," he writes of frenemies.

The good news is there are ways to navigate the rocky relationship. Here are three things Barker suggests you do to cope with and even "bring out the best" in a "bad" person or just someone you typically wouldn't gravitate towards.

Emphasize similarity

To Aristotle, "a friend is another self." There are 56 studies that support this idea, which is called "self-expansion theory." Self expansion theory posits that people are motivated to make friends in order to enhance their own sense of self.

"A series of experiments demonstrated that the closer you are to a friend, the more the boundary between the two of you blurs," Barker wrote in his book. "We actually confuse elements of who they are with who we are. When you're tight with a friend, your brain actually has to work harder to distinguish the two of you."

So, if you're trying befriend someone, Barker suggest emphasizing your similarities. Even with a cousin on the other end of the political spectrum or with combative colleague, you're like to find something you have in common.

The more narcissistic tendencies a person has, the better this works, he notes.

A narcissist loves themselves more than anyone so if you emphasize a similarity, it's harder for them to dislike you.

Emphasize vulnerability

At work or in your personal life, explicitly expressing that you didn't like they way you were talked to or treated by another person can be scary or awkward. But it can also soften the person who administered the blow.

"Two critical points while executing this: voice the importance of the relationship to you and reveal your feelings," Barker writes. "Showing anger will backfire, but disappointment is surprisingly effective. Next time the jerk says something jerky respond: 'That hurt my feelings. Is that what you intended?'"

If the person has any empathy, they will backpedal and re-think their actions, Barker writes.

Emphasize community

Someone who doesn't think about what they say or how their actions are affecting other people might not be used to receiving empathy themselves.

If you remind them that they have a community who cares about them, it might force them to think about the way they treat people.

"Remind them about family, friendship, and the connections you have," Barker writes. The default setting for a narcissist isn't empathy, "so you just need to kick that back into high gear," Barker says.

And if they react positively, make sure you let them know you noticed, Barker writes. "Take a lesson from dog training: positive reinforcement. Reward them for it."

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