I wrote on another thread:
I think it is quite interesting that according to the show’s creators, David and Mary Margaret are NOT the keys to breaking the curse.
On why ‘true love’s kiss’ hasn’t broken the spell on David and Mary Margaret: ‘I think it wasn’t an “aha moment” for them because the curse hasn’t been broken yet,’ Kitsis says. ‘And clearly they’re not the keys to breaking it, they were just the keys to making the savior [Emma, played by Jennifer Morrison].’
My take is that even though I think David and Mary Margaret really do love each, I think the fact that they are both cursed means neither can break the spell. All of the characters from fairy-tale world, except for Emma (who escaped before it took hold), are cursed. My theory is that the hero of the story is the one who is unaffected by the curse: Emma. In fairy-tale world, the curse is broken by the non-cursed party: Prince Charming woke up Snow White, who was cursed when she ate the poisoned apple. Princess Katherine, with the help of Prince Charming, freed her love, Frederick, who was turned to gold by King Midas. Belle nearly freed Rumple by kissing him, until he rejected her advances. As the creators said, I think Emma is the key to breaking this particular curse. My reason for thinking so is she was never under the hold of the curse.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy