When I write a book, it's always my goal to make you love my characters so much that you can't stand the possibility of their getting hurt. I want you to realize fairly early on in the book that my hero and heroine belong together. But I also want you to see that there's a very real, very good reason they cannot be together.
Even as every scene is marching them toward the realization that they belong together and that they could potentially make each other happy, I have an equal and opposite subtext or conflict reminder that proves to them that they can't be together because of that something, that very good reason they cannot be together.
That reason comes with "stakes" attached. If I fall in love with you, give you my heart, you could hurt me. Or, I can't fall in love with you...I have a secret. And once you hear my secret you're going to hate me...and you'll leave. There's a risk involved.
At some point the stakes flip. I now love you so much that it's going to hurt more to live without you. So my characters try to pretend that very real, very good reason they can't be together doesn't exist, but...well, that never works.
They have to face that very real, very good reason they cannot be together. They have to face their demons. They have to find their courage. They have to suck it up. Pull themselves up by their bootstraps and change. Because, you know what? Change or growth isn't about becoming taller or coloring your hair or moving from Montana to Manhattan...It's about suddenly realizing that you are the master of your destiny. You can compromise. You can risk being hurt because the reward is worth it.
And isn't that really the heart of falling in love? It's the realization that the hero or heroine you've found really is worth the risk of making a commitment.