Okay, I actually have a huge soft spot for this dynamic because it plays with so many interesting power imbalances. It's rarely a straightforward 'monster falls for human' thing—the goddess aspect adds a layer of divine, ancient detachment that has to be eroded.
Often, the mortal hero's humanity itself is the key. His finite lifespan, his capacity for flawed but passionate love, his stubbornness in the face of cosmic power… these become virtues to a being who's seen eternity. The romance isn't about him becoming powerful enough to match her; it's about her choosing to become vulnerable. She might start by seeing him as a fascinating pet or a temporary amusement, but his persistent moral courage or genuine kindness dismantles her divine indifference.
I love when the narrative explores the cost. Can a goddess truly abandon her duties for one soul? Does her love doom him or elevate him? The best stories make you feel the weight of that choice, not just the swoon.