“Riptide” by Vance Joy is a song about a guy who likes a girl and thinks about her a lot. While the guy likes her, he thinks that she’ll head down a bad path. When he sings,

“Lady, running down to the riptide
Taken away to the dark side
I wanna be your left-hand man”

it basically means that he’s worried that she’ll go down the wrong path and mess up her life. He wants to be there with her not only because he likes her but because he thinks he can keep her from messing up her life. Unfortunately, his love for her seems to become obsessive when he sings,

“I just wanna, I just wanna know
If you’re gonna, if you’re gonna stay
I just gotta, I just gotta know
I can’t have it, I can’t have it any other way”

He also seems to blame most of their relationship’s problems on the girl because of the bad choices she makes, and this convinces him that she’ll always make the wrong choice no matter what. I like how this song progresses because it shows how liking someone can become something toxic, without the person even realizing it.