I don't get it, not in the lauriver x olicity case. I mean, your anon basically said that lauriver kisses are REAL, so very passionate, sexy and you can feel how they love each other, he/she said REAL. How this does not mean have more chemistry? What I get is that for some people just work that relationship you see the start? Say that lauriver don't have chemistry just because they already loved each other and had history from the start it's not fair.
Well I can't and shan't speak for my anons, but to me I think it's a case of "it doesn't matter how you kiss as long as there's chemistry" - technique is less important than emotion and tension. And yes, a lot of that tension comes from seeing their relationship grow and develop which we never did with Laurel/Oliver or Ichabod/Katrina (save for those too-little-too-late flashbacks) - it connects us with the characters in a way we can't otherwise.
Shipping is, like I said, kind of like falling in love with people as they're falling in love, it's the love of the whole relationship. A couple that starts out with history/as being together is a bit like an arranged marriage - sure, it's possible to grow to love them if everything clicks but it's not the same as choosing it yourself. Those kinds of ships are just a harder sell, for the actors and the writers.
But yeah, the rest you'll have to take up with the anon - I'd give you their name but ... ;)