Why would that kill his mother?

Question

This is about the 1987 film Moonstruck (Academy Awards for Cher and Dukakis), about Loretta Castorini who is engaged with Mr. Johnny Cammareri but then falls in love with his estranged brother Ronny Cammareri.

Mr. Johnny is away in Sicily, because his mother was breathing her last breath. In the meantime, his fiancée was supposed to organize their wedding. She falls in love with Ronny and Mr. Johnny returns unexpectedly early. The climatic scene at the end

From the script:

                                 MR. JOHNNY
                     Well. I told my mother that we were 
                     to be married. And she got well. 
                     Right away.

                                 RONNY
                     I'm sure she did.

                                 MR. JOHNNY
                     It was a miracle!

                                 LORETTA
                     Johnny, I have something to tell 
                     you.

                                 MR. JOHNNY
                     I have something to tell you. But I 
                     must talk to you alone.

                                 LORETTA
                     I have no secrets from my family.

                                 MR. JOHNNY
                     Loretta, I can't marry you.

                                 LORETTA
                     WHAT?

                                 MR. JOHNNY
                     If I marry you my mother will die.

           The Old Man laughs a loud sudden laugh, and then subsides.

                                 LORETTA
                     What the hell are you talking about? 
                     We're engaged.

                                 RONNY
                     Loretta, what are you talking about?

                                 LORETTA
                     I'm talking about a promise. You 
                     proposed to me!

                                 MR. JOHNNY
                     Because my mother was dying! But now 
                     she's not.

For sure, this is an unexpected turn of events, since the viewer saw Loretta's and Ronny's love blossom, so what would happen when Mr. Johnny finds out? But then, Mr. Johnny himself breaks off the engagement. Wikipedia says

When Johnny finally arrives, he breaks off the engagement, superstitiously believing that their marriage would cause his mother's death.

Even if it's just superstition, what is the root of such a belief - after all, breaking off an exgagement is quite a big step to take and the announcement just brought her back to life, so to speak (sounds good, doesn't it?). Is his mother somehow involved (Ronny's remark (see script): "You're forty-two years old, Johnny, and Mama is still running your life."), but if so, why? The old man's reaction (laughter) is also interesting.

Why does Mr. Johnny believe that his marriage would kill his mother?

Answer 1

The response to a similar question here seems logical to me:

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.

TV Tropes has a similar explanation:

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!"

Answer 2

In some cultures, Mother's are prone to melodrama. The concept that Johnny & Ronnies' Mother was "going to die" is an example of such melodrama. It was not meant to be "literal", she wasn't actually going to die. Similar to "oy, you're giving me a heart attack!".