Ah! Madame, I reserve the explanations for the last chapter. - Agatha Christie

“This affair must all be unravelled from within.” He tapped his forehead. “These little grey cells. It is ‘up to them’ — as you say over here.”

“You have been to the Riviera before, Georges?” said Poirot to his valet the following morning. George was an intensely English, rather wooden-faced individual. “Yes, sir. I was here two years ago when I was in the service of Lord Edward Frampton.” “And to-day,” murmured his master, “you are here with Hercule Poirot. How one mounts in the world!”

Jealousy, you know, is usually not an affair of causes. It is much more-how shall I say?-fundamental than that. Based on the knowledge that one's love is not returned. And so one goes on waiting, watching, expecting...that the loved one will turn to someone else.