It all began with a line (Diall)

[Slowly rewriting and editing] Demi Lovato is one of the most famous singers in 2015, she has the looks, she is a true inspiration and a good role model, on the other side we have Niall Horan, a smart guy who has a friend called Harry, and another named Zayn, they're all lovatics- no, I said something wrong, they're lovatics, except Niall, but his friends convinced him to go Demi Lovato's concert with them. Now can someone explain why did Demi Lovato sing a line to the only one who isn't a lovatic in this place?!

'you can't stop this girl from falling more in love with you'