Meeting online is no longer unusual in today's dating scene. But as hookups on Tinder are killing romance, according to Vanity Fair, there are still plenty of couples who thrive and survive in a happy relationship. 

How do they do it? The secret may be in co-dependency, according to a new study. Regardless of how a man and woman met, happiness in a relationship is more telling after couples have had years of an emotionally interdependent relationship.

Researchers from the Rotman Research Institute studied the brain activity of 14 women who have on average  been married for 40 years or more. They also presented videos and tests with their husband or a stranger reacting to negative or positive triggers to record the women's reactions.

The researchers found that women respond better and are more sensitive to their husband's positive reactions than negative ones. In other words, as the women see the person they love the most go through positive experiences, it also affects their own happiness.

"Many of us tend to think we choose partners who help us work on our issues, and of course, that's part of what happens in good relationships," said Duana Welch, via the Daily Mail. "But science increasingly shows that we choose one another for how good they make us feel."

The idea may be ludicrous for some, yet time and again, science has proven that brain activity does affect a person's view on love and relationships.

A study from 2014 showed that seeing a partner's image, one that you're physically attracted to, does bring some calming effects, much like a pain-killer.

Happier couples also tend to look alike as years pass because they tend to adapt the same facial expressions, according to Live Science. Their long history together also brings shared experiences that have left similar lines on their faces, according to a study from 1987.  

Then there's the constant reassurance and acts of love.

Habitually thanking partners and showing appreciation can bring happiness in a relationship, according to a study. The willingness to please partners, which is an act of co-dependency, may also affect relationship happiness. "Instead of just waiting for the other person to make you feel good, you can jumpstart that cycle and take it into your own hands by focusing on what's good in your relationship," said Amie Gordon, the study's head author, according to Psychology Today.

Finally, Happify summed up what scientists have said over the years about relationships and happiness in this infographic below: