There were many things that made Shinobu mad: Tomioka, the monstrosity of demons, how often Tanjirou got hurt so badly he needed to rest for weeks, and Tomioka (he was worth mentioning twice).
But lately, what enraged her even more was Iguro and Kanroji. Those two kept acting like a married couple and between Kanroji's enamored staring and Iguro's jealousy, Shinobu wanted to scream.
But she just smiled. She didn't know how long she would last before she broke down for real, but for now she would continue to just keep calm as if everything was abolutely and perfectly fine.