What's the one thing all happy couples have in common?

A quick straw poll of the Stylist.co.uk office revealed everything from sex to compromise and a sense of humour.

But a relationship psychologist reveals it's none of these factors.

In a think piece for refinery29.com, Dr. Benjamin Le claims that continuing to share new and interesting experiences together - and a partner who helps you to grow - is the key element of a passionate and long-lasting relationship. 

Dr. Le, who co-founded the website scienceofrelationships.com, quotes a 2013 study from Indiana State University titled "Passion for Life".

Researchers quizzed more than 500 people in relationships aged 18 to 92 years old, with a median relationship length of 17 years.

Participants were asked to provide brief statements on different kinds of love they experienced, including "attraction and sensual love" ("your partner and you have the right physical chemistry") and "obsessive and possessive love" ("since being with your partner, you find it hard to focus on the routines of life").

They also answered questions about sharing new experiences and personal growth with their partners. 

As expected, results indicated that self-expansion was lower in longer relationships.

But those people who reported that their partners DID provide outlets for self-growth and new experiences also reported higher levels of attraction and sensual love, and higher levels obsessive and possessive love. 

"In short, having a partner who helps you grow as a person may be an important marker of these forms of love in long-term relationships," says Dr. Lee.

He points out that the study does not prove that shared experiences cause relationship quality; however, one factor is clearly influenced by the other.

And other studies have also demonstrated a clear link between novel activities and enhanced relationship quality; one 2013 paper from the University of California showed that "sharing stimulating activities will enhance marital satisfaction".

And on the flip side, "habituation" - or boredom and routine - may be seen as an obstacle to relationship maintenance. 

"Just because your relationship is long doesn't mean it can't be passionate," says Dr. Le. "By engaging in new, interesting, and challenging activities with your partner, you can have a positive impact on your relationship - even for the long haul."

With that in mind, come take a look at our bucket list of incredible experiences for summer 2015, from alfresco opera to trapeze classes and secret gin gardens. A bounty of novel activities guaranteed... !