Forging friendships as adults is challenging. It's even tougher to find two couples where everyone likes one another. Picture: Pexels

Washington - A few months ago, my friends - a colleague and her partner, who share my boyfriend's affinity for cute-goth Instagram illustrators and metal - moved to New York. We had planned to move there, too, but life has kept us in Boston for at least another year. Their move has left a void in our social lives.

We have other friends, couples and singles alike, and we love them. However, the loss of our ride-or-die couple friends has been a hit we were not prepared for.

Forging friendships as adults is challenging. It's even tougher to find two couples where everyone likes one another. Why do couple friends seem so imperative to our romantic lives? Why are some better than others? And how do we make new ones?

"Getting together with another couple can make your partnership seem stronger," Geoffrey L. Greif, a professor at the University of Maryland School of Social Work and the co-author of Two Plus Two: Couples And Their Couple Friendships, told me. "You get to see your partner in a great light - they're interacting, having fun, happy, that makes them more attractive. That's the upside."

Greif and his writing partner Kathleen Holtz Deal explored the dynamics of couples and their couple friends, and found that heterosexual relationships benefited best when they were able to connect on a deep emotional level with another couple.

They found gender roles within romantic relationships are more fluid when intimately interacting with a fellow couple. "Research tends to say men get drawn into conversations and face-to-face interactions he might not feel comfortable doing on their own," he said. "Women could then use that time with another couple to get her husband or partner to talk about things he wouldn't otherwise."

Greif and Holtz Deal placed couples in three categories: seekers, extroverts actively searching for new social relationships; keepers, those who feel fulfilled within the confines of their relationship and are happy with an intimate group of confidants; and nesters, introverts who prefer to stick to a party of two. Greif, who identifies as a seeker, and his nester wife often find themselves negotiating to form meaningful friendships with other couples.

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"She pulls me in a bit, and I pull her out a bit, and we meet in the middle," he explained. "Knowing those roles and having that discussion should give couples a language." And in developing that social contract, the relationship deepens its bonds.

The No. 1 reason for couple friends: having someone else to talk to.

"The biggest issue for every couple, across all the books, is what to do with all the time," Greif said.

No. 2? Having something to talk about.

"Take people like my parents - they're interesting people, but their conversations are very limited to their own perspectives, in that, they're not expansive," says Laurel House , celebrity relationship coach. "If you have interesting couples in your life, you have different perspectives, and conversation can expand, and you feel more interesting, and more confident."

Improving conversation between couples was a main goal for Cory Nitschelm, founder of Coupler, a soon-to-be-released swipe-based social app for double dates.

"We believe making couple friends strengthens a relationship from an external and an internal perspective," he said. "That way, it's not just the two of you doing the same things, stagnating."