Although high school students may think popularity is of the utmost importance, a new study found that it may not lead to long-term happiness. What mattered more, according to the study, is whether the teens had close friendships they maintained over time.

The article doesn't say specifically, but I'd assume from what I read that it is the latter. That as an effect of trying to create shallow friendships they become less happy than those who don't.
Ofcourse, there's no way everyone can fall under the same category so it'd probably be close to impossible to have an 100% accurate cause.
For me personally, I'd probably fall under the latter. It's not that I was unhappy in school or that I wasn't able of making close friendships, I probably had 3 or 4 close friends and was in general friends or atleast on good terms with everyone else in school.
It was just a bunch of factors that made me not maintain those close friendships, I'd rather not say that not maintaing my friendships was a result of them being shallow or weak tho, it just happened by being naivé and lazy.
And then stuff just spirals when you're set in your ways.

I'm not a big fan of this article, it just feels like it painted too much with the same brush, difficult topic to be sure but a more in depth presentation would be appreciated. To me this research can pretty much be boiled down to; Unhappy people don't make friends and people who don't make friends become unhappy. As it's presented right now atleast.

/r/science Thread Parent Link - abcnews.go.com