What makes you say either 'hey I want to RP with this person way more' or 'hey I don't want to actively RP with this person anymore'? Can be IC, can be OOC; just curious about the spectrum.
For me, I've found my breaking point is often centered around pressure to TS. This has happened before and happened again recently to me. Char was in a long term relationship with another character and the other player made some OOC comment about not getting enough TS. Despite their apology when I called them out on it, I found myself liking the player less and less for other things I would have normally ignored and I eventually asked to end the relationship IC. I couldn't get passed the 'I'm not getting enough TS' mentality.
Something that will almost always make me want to RP more with people is probably pretty common, and that's level of interaction in a scene. Are you drawing me into something my character can contribute to? Are you asking questions that they can respond to? Are you physically doing something that they can play off of? Are you handing out plot like it's going out of style? That sort of engagement makes me want to play with that person more!
So! What's your turning point?