Every once in a while, Yukari likes to spoil Ran with a kiss or two, like the good master she is.
It's always a lot more fun to do it when she least expects it, catching her off guard.
At one point, Ran remained constantly vigilant for several weeks, ready to receive her master with a dignified face. The days passed, and passed, and she waited and waited...
Until she grew impatient (and lonely) and just straight up asked for her kiss.
Yukari giggled in satisfaction, and then happily complied.