What makes romance romance? I'd say the requirements are

1) Romantic love as a central story motivation and the pivot around which story ark revolves. Central conflict should be between love and other forces (for example duty, societal expectations and so on).

2) Romantic love is accepted as a factual, existing thing, which brings satisfaction and meaning to people's lives.

3) Emotional justice.

Going strictly by rule 1, many romance books would end up not fitting my definition. I just recently quick-read through a contemporaty romance, where a handsome police is assigned to protect a smart and pretty surgeon from a potential threat. They develop a crush on each other while trying to solve the mystery of who's threating pretty surgeon. Many scenes of suspense ensue, and the mystery/thriller plot is solved quite acceptably, but there are no emotional obstacles on the pair's path to HEA. They do end up together, blissfully happy, but their romance was actually quite incidental to the story. So I did not find in it what I want to find in romance literature.

Of course, some books that are not usually considered romance end up with that label, if following my rule, but I'm sure they won't mind.

Rule 2 filters out all "real literature", which explores love as a social construct etc. While in my daily life I may not believe in Love as an all-powerful, all-important force of nature, which anything, everything and everyone must obey or suffer terribly, in my entertainment I want it just that way.

Rule 3 usually takes the form of Happily Ever After, where protagonists are good people, persevere through difficulties and eventually triumph and find happiness in each other. But personally I'm just fine with shitty people being given the chance of happiness but screwing things up and ending up unhappy – a common outcome for antagonists or some side character in traditional romances. Emotional justice is what makes romance a "happiness machine".