Just thought that Keiko's been quite sadistic towards Ran this whole time. This reminded me of the chapter 14, and I thought that may be she was quite hard with her ex too? Perhaps she really needs this? Is it coming from her mum beating her in the childhood or am I reading too deep into the subtext?
And Ran's like the opposite - seems to be grown up in a proper family, never got hit by anyone, yet the first thing she expects from Keiko is "extort me", "beat me up like a sandbag", "kill me in cold blood" - so iMaginative. Is this what actually excites her? These days at least she seems to quite literally enjoy being spanked by her dear senpai, LOL. And this makes Keiko quite bewildered in return: "How come you look like you're enjoying it..."
This is in a sense the reason why the look like a really good couple. The behaviour of one excites another, but the response to this excitement calms down both and prevents from severely hurting each side.
Hell, all that's left now is marriage.
Komatsu Ran - sounds legit, right?