Calvin Harris - We'll Be Coming Back Lyrics Meaning

It's about a couple who slowly drifted apart, not really knowing what happened. With ambiguities and distance separating them, Basically the man misses the woman really bad, and his thoughts of her are haunting him.

"Someone had to draw the line" is the point where he got so obsessed with her that he was forced to cut ties with her, even if he didn't want to, it was, ironically for himself.

"We'll be coming back for you one day" really refers to the thoughts about her, and the thoughts are making a statement, telling him that "we'll be coming back for you one day" even though he thought he got over her at one point or another.

"I don't even care if I know you, out of our minds, trying to leave it all behind" is really a warning to her, he doesn't know if he can control himself anymore because he really wants to be with her.