Board Thread:Show-Related Questions and Answers/@comment-26116863-20160926025050/@comment-25375217-20180717190328

What was clearly stated about TLK:


 * Both parties must be in love (source: Ruby in 518, various implied occurrences)
 * This also apparently spreads to when one party does not remember they are in love, as seen in 116. Despite this, Will was able to break Ana out of thinking she loved Jafar with True Love's Kiss in W13.
 * Both parties must believe in magic (source: Regina in 710)
 * Despite this, Emma's kiss worked on Graham. Although since this did not break the Dark Curse, this could be a case of something besides True Love.
 * The curse must be unwanted (source: Belle in 502)
 * This apparently includes when the curse used to be wanted but isn't anymore, as Belle and Gold couldn't break the Dark One curse in Season 7.
 * If both parties are Dark Ones, True Love does not work (source: https://twitter.com/InkTankGirl/status/666120732212989952)
 * However, this seems more like retroactive justification to me than a legitimate reason. A more logical retroactive justification would be the aforementioned excuse for Belle and Gold being unable to break the Dark One curse in Season 7.

It's also implied by cases like David kissing Snow on the forehead that it is possible to kiss someone without triggering True Love's Kiss.