Why did Taylor and Calvin really break up?

There's just one question on everyone's minds about the Taylor Swift/Calvin Harris break up. Why?

News of the split came as a shock to the world, but the couple's relationship had reportedly lost its spark months ago.

RELATED: Taylor and Calvin's love in photos

RELATED: Taylor Swift and her DJ boyfriend Clavin Harris have split up!

"Taylor's heart was more in it then he was. He started to lose interest over the past few months, but really tried to not just break up," a source told E! News.

The source also added that Calvin "100 percent did not cheat", and it was just a case of their relationship fizzling out.

They added: "Taylor was really disappointed in Calvin and felt like he wasn't there for her. The relationship wasn't progressing the way she wanted it to. She felt like he wasn't taking it seriously enough and wouldn't come through for her when she needed him most."

Boo-urns.

RELATED: Calvin Harris Shows his face for first Time Since Car Accident

TMZ reported this morning that the couple called time on their romance after 15 months of dating.

Neither of the stars are yet to publicly speak about the break up.

Want more celebrity, movie and music news worth sharing? Follow us on Facebook, Twitter, Tumblr and Instagram.