It's the title of the book, so it comes as no surprise that the song Diana sings is symbolic. Check out the lyrics:
"It was hard love, every step of the way, Hard to be so close to you, so hard to turn away, And when all the stars and sentimental songs dissolved today, There was nothing left to sing about but hard love. So I loved you for your courage and your gentle sense of shame, And I loved you for your laughter and your language and your name, And I knew it was impossible, but I loved you just the same, Though the only love I gave to you was hard love." (16.42)
We know that she chose the song for John because—you guessed it—he's dealing with a hard love in his life. He loves Marisol, even though he doesn't want to, and he just can't seem to shake his feelings for her. John gets that his love for her can't be explained, and it doesn't make sense, that just as the song says, it's "impossible." Still, it's a big deal to John because he's never been in love before.
The song goes on to say that "Love is never wasted, even when it's hard love. Yes it's hard love, but it's love all the same, Not the stuff of fantasy but more than just a game. And the only kind of miracle that's worthy of the name, For the love that heals our lives is mostly hard love"(16.44). Did you notice that first line? It tells us that even though John is down in the dumps about Marisol not feeling the same way, his love for her isn't worthless. It's not something that he'll look back on and regret, it still heals him even if it hurts him a bit along the way.
This is a pretty major deal. For cynical John to admit that love is important in his life helps us understand how much he's grown over the course of the book. He's not the same jaded guy he starts out as anymore. He's open to love for the first time—even if it hurts—and the song helps us see that.