I wonder what their story is. Why suddenly the pink girl left?... My theory is at the beginning they both are bored individuals who just met by coincidence and bonded over their similarities. At first it was full of fun and excitement. But at some point we can see the blue girl stopped smiling as much even when together. Same goes for the pink girl.
My guess is they got used by each other presence that its not as exciting as they used to be(?) So at one point the pink girl realize they might get bored of each other sooner or later. So she ran away out of fear that her guess might be true(?)
Something like "I dont want this feeling to fade so let me just end this so that it can stay like that" OR "Did I bore her? Should I left before she does?"