Notes from a Deep Story workshop presented by Carol Hughes (adapted and reprinted with permission):
In a romance, the Protagonist is the character who is going to undergo the emotional growth in your stories.
You can actually have 2 different protagonists -one character is the protagonist of the love story while the other character is the protagonist of the overall story. The Protagonist is the one who must learn to fall in love/be willing to trust enough to fall in love. And the other character is your Love Interest.
Since women are the predominate readers of romance stories - and since your reader becomes your protagonist inside of your fictional story world - the romance genre stands out once again for being unique in that your heroine is usually who helps the Hero/Protagonist to open up and fall in love.
Why are romance stories different like this in having the heroine be the inside character that your readers ID with inside of your story world? EASY - YOUR FEMALE READERS WANT TO BE THE ONE TO MAKE YOUR ROMANTIC HERO FALL IN LOVE WITH THEM!! And they usually can't do that when they are the one who is the protagonist in your romance.
But please note - this is not set in stone. There are wonderful and successful romance stories where your heroine is the one running away from love and is the story's protagonist. Just remember the story Protagonist is the character who undergoes the emotional growth in your stories.