Hero and Heroine Don't Understand Each Other

It’s not just the English and Americans who are separated by a common language. The younger the fledgling couple is, the more likely they will miss the cues or clues to understanding each other’s motivations. Empathy and understanding grow with experience, which usually translates into more years on the planet.

Many married couples come to appreciate each other like a fine and aged wine.
Your young hero is built to save the world. Therefore, he’s especially prone to see compromise as weakness. This can ruin a budding relationship in a novel. Think back to all the fights you had with that wrong-for-you prospective mate. Just perhaps, if he or she became thoughtful for only a short while and reflected on the inner motivations of his or her candidate for marriage, a different outcome might have presented itself.
But, it’s dangerous and difficult to write the young hero as precocious in this way. Something about realism. That’s why many use the sidekick/mentor/wizard as a guide to a HEA (happily ever after). Besides, it’s funnier to see a young couple who the reader wants to see succeed; stumble and then the reader can’t wait to see how he/she recovers.