There’s no formula, and it depends on the quality of writing, the familiarity with the author, the depth of emotional involvement and certainly the hotness of the buildup and the sex.
And yet it is simple. Do I like the story?
A Like is certainly more likely from me in certain genres or situations that tend to appeal to me anyway, in which I may be more willing to overlook a few flaws if I find myself getting turned on. It’s also more likely if it’s a familiar author whose works I have Liked in the past.
It’s less likely in genres that tend to be active turnoffs for me (dom/sub, ), and yet I have Liked quite a number of stories with subject matter that tends to repel me, because they were very well-written. In other words, I still like the story despite the subject matter.
The biggest deal-killer for me in terms of Liking a story is poor quality writing with significant grammatical errors (such as writing “discrete” when you mean “discreet,” a current pet peeve), repeating the same thought multiple times or using the same word for something over and over again when plenty of synonyms are available. That makes a story much harder for me to Like, though not impossible if it’s incredibly hot otherwise.