3 reasons why your spouse shouldn't be your best friend

• Partners are often referred to as "best friends" in wedding vows and Instagram posts.
• Author Rhaina Cohen argues that puts too much pressure on one person to be everything.
• Making your partner your best friend is a new phenomenon in human history.

Whether you've just been to a wedding or your Instagram feed, you've probably encountered someone's significant other being referred to as their best friend.

While close friendship is a core part of all healthy romantic relationships, it's also really important to have friends — including your best friend — be separate from the person you share a bed, mortgage, or child with.

In her new book "The Other Significant Others," author Rhaina Cohen argues that centering your life around friendships rather than solely romantic partners won't just make you happier — it'll safeguard you from loneliness should you split up.

Cohen said a formative experience was watching a relative keep entering long-term relationships where the romantic partner "was a confidant and career coach and intellectual conversation partner and roommate," only to be left mostly alone when those relationships dissolved.

"That illustrated one of the real risks of putting everything on one person and not having other kinds of connections," Cohen said. "If things go south, as romantic relationships can, then you put all of your eggs in one basket."

She shared the reasons everyone should have all kinds of meaningful friendships and relationships outside of their romantic partner.

1. It's how people have lived for most of history

Cohen said that viewing partners as best friends is a relatively new phenomenon that's been popularized over the past 50 or so years.

Historically, people "found some of their greatest emotional intimacy in their same-sex friendships, partly because they weren't expecting to find that in marriage, which had been much more of a practical relationship," Cohen said.

Investing in close relationships isn't just scientifically proven to help you live longer: it's what your ancestors did, too.

2. You have more potential for growth

When long-term couples were asked for their best-kept marriage secrets, having separate lives (including friendships) helped keep the relationship fresh.

Much of that has to do with the novelty and new experiences that friendships offer outside a romantic partnership.

"Having more close relationships in your life opens you up for more people to learn from and to influence you," Cohen said. A friend might introduce you to a new hobby, influence your career, give you life-changing advice, or even become family.

"There's a lot of growth and discovery and connection that's possible when you are looking toward multiple close relationships in your life," she said.

3. Your romantic relationship will be happier

Having high expectations of a partner to be your best friend, closest family member, and a hot lover can create more conflict in your relationship — especially if you're always around each other.

But investing in community actually strengthens your bond to your partner. "Having more people in your life can create more space to be excited to see each other," Cohen said.

At the same time, she said it can "also reduce the pressure for one person to always be the right person for every kind of need and desire that comes your way."