When you start a romantic relationship, you go in with some idea that you and this person could go the distance — otherwise why would you waste your time?

Everyone wants to find the perfect formula to make a relationship last forever, but because every couple is different, a formula doesn't quite exist. That being said, there are some important traits that all successful couples share. Therapist Kimberly Hershenson told INSIDER what she feels long-lasting relationships have in common.

You trust each other

It's a big deal if someone puts enough trust in you to devote time to you and open up to you. If you're going to be with them for a long time, it's important that you show reverence and respect that bond.

"It is difficult to be vulnerable and share problems with others," said Hershenson. "When a partner is able to open up to you it is important to not break their trust."

In a relationship, trust is everything. If you feel comfortable and confident that your partner won't do things to hurt you and is not going to cheat on you, then you may have a winning team.

You support each other

The world can be a scary place, and one of the best parts about having a partner is having someone on your side when the going gets tough.

If you feel like your partner believes in your dreams and is there to listen when you need to talk something out, that's a big part of a long-lasting relationship.

"Asking your partner how they are doing sometimes without even sharing your own personal issues allows you to be completely available to them," said Hershenson. "Listening to others' problems and lending an ear is a good way to "get out of your head" and let your partner know you are fully present to listen to them."

You hold each other accountable

Partnerships mean being kind, but it also means speaking your truth when they do something that upsets you. Staying quiet doesn't lead to happiness, so knowing when to call your partner out in a healthy way will only make your love stronger and last longer.

"If your you or your partner is upset with the other person talk it out without getting defensive," said Hershenson. "Acknowledge what your part was (even if it was simply upsetting them) and discuss what you could do differently in the future."

If you are both able to admit when you're wrong, it will cause your fighting time to be cut down and will prevent simple misunderstandings from turning into something much bigger.

You show physical love

Though it's important to show your partner love and support through your words, keeping up some form of physical love is important. This doesn't just have to be sex — making sure to give your partner a kiss when you leave for work or holding their hand while running errands are other, more subtle ways to connect physically.

"Whether it's a kiss hello or goodbye, snuggling on the couch or holding hands; even non-sexual touching builds connection between partners," Hershenson said.

Even if it doesn't come naturally, keeping up a routine of showing physical affection can make all of the difference.