It's easy to be patient alone. You read about calm, sit somewhere quiet, feel it settle, and believe you've got it. Then the queue stops moving, someone talks over you, three things go wrong at once, and it's gone.
Knowing about a quality, and even feeling it in a quiet moment, are real steps. But they aren't the same as living it. The quiet half-hour is the study; the rest of the day is the exam, and it doesn't announce when it starts. One sharp word, the tone, the volume: that's where you find out how much of the calm was yours. The gap between who you are on your own and who you are in the thick of things is usually wider than you'd like.
That gap is also where calm starts to matter to anyone else. What you're full of is what you hand out. Someone carrying a lot of pain can't help telling you about it. Someone steadier passes the steadiness on without trying, in how easily they let things run late.