PS: I can't speak for everyone, and I myself am not a fan of the revenge and slap/kiss genre, but I think part of the appeal of revenge-based stories and slap/kiss dynamics is the drama that comes with it and that is pictured on-screen. Some people might enjoy a really intense, emotionally-provoking drama vs your light, warm and fluffy shows. Hence, having a character driven by revenge motives tends to bring a lot of emotional intensity and a lot of crazy dramatics on the screen, which may appeal to people who are in the mood for a loud, in-your-face, dramatic show. The slap-and-kiss dynamic enhances this perception too which is why a lot of revenge-based dramas tend to also have the slap/kiss trope in them.
I find that a lot of Thai drama shows tend to like some over-the-top scenes or characters in them and it doesn't matter whether they are revenge-based slap/kiss shows or not. Quite a lot of the romantic-comedy shows or just pure romance-driven shows tend to always feature at least some overly-dramatic, one-dimensional caricature-ish character(s) who is/are there to either stir up some drama or to create comic beats/ moments. The villains and crazy second-leads (usually females in heavy make-up and eye-catching clothes) usually fulfil this role, but you also sometimes have some silly side-characters who are very loud and exaggerated in their portrayals too. I think this is one of the ways the show does to keep some aspect of 'intense' and 'drama' which Thai viewers like in their rom-coms or pure romance shows (i.e. non-revenge slap/kiss). Even the historical/ period shows are not exempt from these over-done dramatic characterisations.
Another thing I've noticed is that there is a lot of Thai action-based shows too i.e. mafia or military shows. Maybe this is because again, this sort of action-based shows tend to bring on lots of suspenseful situations and circumstances fraught with danger/ excitement/ anxiety. Again, this appeals to audiences who like their shows intense and exciting. And yes, there is always a romance built into these shows too, which lends another aspect of 'intense' and 'exciting' to viewers.
In short, I suspect the Thai audience like their shows loud, obvious and energetic with generous helpings of intensity and excitement, so if you like your shows like this too, well, I think you've probably just slipped down the rabbit hole and arrived at Wonderland! :P
