When Odysseus strings his bow, that none of the suitors beforehand have been able to string, he then plucks it like a harp, sounding a note. After that, Zeus gets involved, sending a thunderclap as a sign:
In the hushed hall it smote the suitors
and all their faces changed. Then Zeus thundered
overhead, one loud crack for a sign.
And Odysseus laughed within him that the son
of crooked-minded Kronos had flung that omen down.
The Odyssey, chapter 21: "The Test of the Bow", translation by Robert Fitzgerald (1961), lines 384–388
Why does Odysseus laugh at this sign? What's so funny here? Why is a laugh an appropriate response in this scenario (or is it)?