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There is an implication in the film that Johnny’s mother doesn’t want him to get married. He’s a mama’s boy, and she wants it that way. This relationship is not unknown among Italian mothers and sons. So Johnny got up the nerve to propose to Loretta because he knew his mother was dying and wouldn’t be around to run his life any more. In the climactic scene in the Casterinis’ kitchen, Johnny tells Loretta in so many words that he proposed to her in the first place only because he thought his mother was dying.
You may also recall a telephone conversation in which Johnny, in Palermo, tells Loretta that he hasn’t broken the news of his engagement yet. Well, when he finally broke the news, his mother got out of bed, got dressed, and cooked a meal for everyone in the house. She got well out of spite, to make sure her son wouldn’t marry.
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My Beloved Smother: Johnny's mother. She's dying — til she finds out Johnny's found a Parental Substitute in Loretta. She then has a miraculous recovery and eats a dinner "that could choke a horse". Johnny breaks off his marriage with Loretta because she'll die if he gets married. Ronnie scoffs, "You're 42 and she's still running your life!"