Why do people laugh softly when they're upset?

Question

for example i read this book and this woman found out her husband was cheating when he was supposedly on a business trip and then she said to her friend she shouldve known and then she laughed softly.

why do people laugh softly when they're upset?

are they laughing at irony?

Answer 1

They can't believe whatever there sad about happened and so they just get worked up and laugh.

Answer 2

It is a social convention employed to disguise one's true feelings, and prevent the utterance of malicious or vindictive rhetoric. (I think).

Answer 3

They're laughing at how ridiculous the situation is

Answer 4

it's nerves.