Though this song is pretty upbeat, I find it to be pretty sad and one I can highly empathize with. It's about a guy who wants a girl back after breaking her heart some months earlier. All he had to do was stay and it would have been *so* easy-she wanted to be with him and she would have worked so hard at their relationship. But he said goodbye and she's had to deal with the fallout from that, trying to rebuild herself. I think the melody can kind of disguise that heartbreak but reading through the lyrics really brings it home for me.
One interesting audio quirk is the way she says "stay" throughout the song. Apparently it is inspired by a dream she had about an ex-boyfriend where that was the only word she could say and it was pronounced that way. I have seen some people who found it annoying but I think it's helps make the song stand out. You'll need to listen to it for yourself to make a decision :)
Next Week: "Shake It Off" - the very well-known first single.