In a romance novel, the main conflict is an emotional one. The hero and heroine both bring to the story their own personal backgrounds, personalities, characters and feelings. Inevitably, they clash (if they didn’t, there wouldn’t be a story!).
This may be because of their own individual experiences. For example, the heroine may be so wrapped up in her career that she thinks she hasn’t time for any relationship with a man, the hero may have had a bad experience in the past that makes him wary of trusting anyone.
But if the whole story involves these two discussing/agonising about their own personal emotional issues, it might as well take place in a psychiatrist’s office! So there has to be conflict between them, as well as within them. This could arise because one of them upsets or angers the other in some way, or because they are fundamentally opposed to each other about something that matters to them both.
This is where the external conflict comes in which can intensify the emotional conflict. The external conflict comes from the plot, the situation in which they find themselves, or from other characters in the story.
Creating a story involves interweaving their internal conflicts with the external ones, and piling on the pressure with a gradual build-up of conflict, tension and seemingly insuperable odds. The reader knows they will get together for the HEA ending, but can’t see (just as the characters themselves can’t) how it can all be resolved.
A Happy Ever After ending involving something coincidental, or contrived, or instigated by another character, leaves the reader dissatisfied. The hero and heroine have to resolve their own conflicts, in a way that is realistic and satisfying.
I can do no better than to repeat here a quote I once wrote down (with apologies that I cannot quote the source): “The best romances are built around a complex emotional conflict that's played out in an equally interesting and tightly connected context - one that forces the characters to deal with each other and their issues.”