Have them argue about something small (like music or a text message) right before they deal with a life-altering prophecy. It makes the relationship feel real.
In high-stakes destiny plots, the romantic interest often serves as the only thing keeping the protagonist grounded or "human."
"If I’m meant to burn out to save the world, I want to spend my last few sparks on you." 3. The "Slow Burn" Awakening
A major trope in teen destiny stories is "Choice vs. Fate." Use dialogue where they question if they actually like each other or if they're just "supposed" to be together.
This works best when the characters are on opposite sides of a prophecy or a supernatural divide.