Why was Jenny unhappy at the beginning of the conversation?

M: Why? Jenny? You look unhappy.
F: I lost my temper at an old man just now, and I feel terrible.
M: Oh, you shouldn’t have done that. But why did you lose your temper?
F: Lack of patience. He was counting out the coins, one by one, with his fumbling fingers. I was filled with exasperation at the checkout point. Finally, I said to him: can’t you count faster? Not everyone has as much time as you! He flushed. Suddenly I realized just how cruel I had been. Because I, too, could become dependent on the kindness of strangers one day when I get as old.
M: Did you say sorry to him?
F: Yes. But I just wonder, how I can become more patient. You know, when you are impatient, you are apt to be rude. And such behavior is counter-productive, making people angry or stubborn or uncooperative.
M: Yes, I sometimes act impatiently, honking my horn the moment the light changed, speaking sharply to someone slow to understand. We really should learn various approaches for calming ourselves in stressful situations. I have even found some books to help me. There is some useful advice.
F: Can you tell me something about what you have read in those books?
M: Well, one of the books says that we should allow for marginal error.
F: What does that mean?
M: That means it is better to provide a margin for error. Impatient people don’t like to waste time, so they cut things too closely. For example, if you are going for an appointment, you should plan for delays.
F: Well...