We know the book is about love—it's called Hard Love, after all—and we also know the love that John has for Marisol is tricky because it can't be reciprocated. That's where the hard comes in. Or part of it anyway. John has a hard time loving anyone or anything, so that his feelings are unrequited for Marisol is really only one part of his difficulty.
Diana spells John's journey out for us when she sings a song called "Hard Love" at the zine conference. She even tells John that she sang Bob Franke's song for him, and the lyrics are included the book. Check it out:
"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)
The song says that even if love is messy, complicated, and annoying, it's still worth it. So not only is John's love for Marisol worth it even if it's hard, but his shift away from being closed off to those around him in general is, too, even if he gets hurt a bit along the way. The title makes sure we catch this message, especially since things end on an awkward note between John and Marisol. The love they share—hard as it may be at times—helps heal them both, even if it leaves them with some emotional scabs, too.