Why Selena Gomez and Demi Lovato taking a photo together is such a big deal.

Overnight after Demi Lovato and Selena Gomez hooked arms at the InStyle Awards, looked down the barrel of a camera and smiled, news headlines came in thick and fast, the innocuous photo carving its own space in the news cycle.

Without context, the frenzy the photo created would be confusing, perhaps a little baffling. In context, the duo coming together on a red carpet for the first time in two years is the most public example of the complicated politics of childhood friendship. And it’s for that reason their most invested fans made their delight known on social media.

Because in the 17 years Lovato and Gomez have been friends, the trials and tribulations of their relationship – their feuds and their reconciliations – have played out publicly, without filter and with fans playing along at home.

So what it is about them that has the world’s interest so piqued?

Perhaps it's our insatiable appetite for the intricacies of celebrity life, or perhaps it has more to do with the recognition that no one is exempt from the complexity of growing apart with someone you grew up with.

Regardless, it makes for quite a tale. A fluffy - occasionally petty - tale, and one that might have you wondering all the way through why it is you care so much. But perhaps that is the pull of celebrity; we all care, and it doesn't entirely matter why.

Meeting on the set of Barney and Friends at the age of seven, the duo worked together from 2002 to 2004. In 2008, they starting vlogging together as YouTube begun to make its mark. In 2009, they starred in a movie together called Princess Protection Program and in that same year, they were at the forefront of a joint spread in People.

But 2010, a rift was evident, with neither keeping it quiet. When Lovato was asked by a fan, “How’s Selena?” she responded, “Ask Taylor.” Taylor, in this case, is Taylor Swift, long-time friend of Gomez and creator of the now infamously dissolved squad of stars.

In the same year, Lovato was remarkably cutting about the state of their relationship, telling Girls Life, “We’re both busy, and I wish the best for her. True friends don’t let their friends or family be mean to you. If you can’t trust somebody, you can’t be friends with them.”

However, in the months after she made these comments, Lovato went to rehab, and publicly thanked those who were by her side.

“Selena… she called me crying when I was in there and was just so worried,” Lovato told Televisionet. “I was like, ‘Look, I should be the one crying, it’s all good!’… But she was so worried and ever since then we’ve gotten closer.”

Listen: When saying no to a friend ruins a friendship. Post continues... 

By 2011, they were back appearing together on red carpets, by one another's side a the Kids' Choice Awards. In 2012, at the same awards show, fans noticed the duo avoided each other and went home in different directions.

But before you thought it was ironed out and cut and dry, Lovato came along with this quote in 2013:

"We’ve been through a lot together, we’ve had periods of times we here we grew apart and we just didn’t really talk,” she told MTV.

“Now we are in a place where I think we realised that life is so short and that when you have people in your life that love you so much then, you should just always be around them. She’s going to be a big sister, and I want to be around for that. Everything about her, she has a great soul, she’s talented, she’s beautiful, and she will always be really, really close to my heart.”

In 2014, things got a little more complicated, with Lovato conspicuously unfollowing Gomez on Twitter and Instagram.

“I think it’s just one of those things where people change and people grow apart,” Lovato explained on Watch What Happens Live at the time.

And yet, a year later, Gomez revealed they were back on good terms.

“We went to a dinner, I think it was two weeks ago,” Gomez told E! “We were at the dinner, we were talking about her record, my record, just kind of catching up… I love her. She’s like family at this point… It’s just nice to be able to see people that you grew up with succeed."

This year, in the wake of the release of Lovato's documentary Simply Complicated, Gomez wrote to her on Instagram: “This was beautiful. I’m so happy for you. You always continue to be bold and real. I wish more people were like you. Love you.”

And this week? Well this week they've posed together for the first time in two years, re-affirming their bond, their loyalty and how the most realistic of friendships aren't always the smoothest.

And our interest in their relationship might say more about us, and our own, than it ever will about them.