In romance, internal conflict is crucial. It's the overriding thing that drives your hero and heroine apart, especially right after they experience some kind of connection with one another (a love scene, or other emotional "click" that starts them on the path to love). If you don't have internal conflict to pull your hero & heroine apart, your reader will quickly get bored and shut the book, no matter how many exciting gunfights or chase scenes or tense hostage situations your hero or heroine have to endure.
So, what's internal conflict? It's the conflict within your character that prevents him or her from automatically reaching a happy ending - the reason he or she can't commit. Perhaps your heroine's been burned one too many times by a macho alpha male. And what's your hero? A macho alpha male, of course. Only she doesn't know until the end of the book that Macho Man is hiding a soft streak. That's your heroine's internal conflict: distrust of alpha males due to past pains. It's her baggage, preventing her from a happily-ever-after with your hero.
Now, maybe your hero has a secret: he's published a book of poetry, and his macho buddies will laugh him out of town if they find out. He can't have that - it'll ruin his tough reputation. Aha! Motivation to be a macho alpha male - just the thing the heroine hates. His need to hide his soft side from everyone - including the heroine - is his baggage. Internal conflicts of your hero and heroine work so much better if they conflict with one another. Your reader will eat up the book just to find out who wins the emotional tug-of-war. (Of course, in romance, we're guaranteed a happy ending, but the excitement and conflict of the journey makes us forget that.)
Next time, we'll build on these two characters by adding external conflict.