"Please don't ask me."

I think it would likely have been a combination of things. Obviously, the children's feelings would be one, though I don't know that he would directly say that. It might be something like his refusal to send them off to boarding school, and then saying it just wouldn't work because they don't share the same family values.

Politics would have been another reason. In the stage version they break up because Elsa is more clearly interested in maintaining her business interests by going along with the Nazi regime. That would have been abhorrent to Georg. Her involvement was different than Max's 'go along to get along' hope of flying under the radar to avoid personal harm.

I can't help feeling that Elsa's life is so rooted in Vienna that she would want to spend much of her life there - she would still have her house and friends there. Georg has already said how he didn't like the parties etc, so that could become a bone of contention, because it would be difficult to transport the kids back and forth all the time. Liesl might like the city life, but I can't imagine the other kids going for it.

Do you think the kids might have played a trick on Elsa to make her go away in the way they played them on the governesses?!

I agree that Elsa would want to spend most of her time in Vienna. Though, I'm sure she wouldn't want the kids there all the time. I guess that is when the boarding school topic would be brought up. I would think that where they would spend their time, where the children would be enrolled in school, and how often they would be away from the the children would be topics they would have discussed before getting engaged. Though, maybe all concerns were swept away with "oh, we'll figure that all out later. I'm sure we can come to an agreement that suits everyone." kind of thing. And Georg, being distraught over the loss of Maria, just shook his head and agreed (while downing a whiskey). 

His question is so direct and out of the blue that she doesn't have any time to consider a better answer; I tried to think of what I would say if I were her (knowing the situation would arise), but there isn't a lot she could say - as it is, what she says works well to avoid the subject.

I began to think what he could have said to be a little kinder and to encourage her more, and which might have had a better effect:

"We really missed you being here"?
"The kids felt their choir had lost their most important member..."
"The abbey must have had an important need for you, but we also need you here..."
"I was sad when you left and hope you will stay longer this time?"

Accepted that much of the drama of the film is created by his character being so unapproachable, but he doesn't make it easy for her!

I'm sure he was deep in thought about how he could persuade Maria to stay, but I also think her return has brought into focus the gulf which is increasingly apparent between him and Elsa.

I was reading an article the other day which talked about quite how pre-eminent Vienna had become since the First World War (ie 1920s onwards), in terms of its vibrant and diverse culture of the arts, music, literature, psychoanalysis etc, which was encouraged by government subsidy, making it widely available. This encouraged writers, musicians, artists etc and the building of many concert halls, festivals etc, with a correspondingly sophisticated population (for those who could afford to indulge in it).

I suppose I am thinking of Georg embedded and isolated in Salzburg, which although it has a culture of its own, will not be close to swinging Vienna; there must have been a big culture clash therefore between the two - Georg the Country Boy with Elsa the doyenne of Viennese social life. I know this has been touched on before, when he says he doesn't like Vienna parties, but I don't think we have really talked about different their interests; I also think of him as a submarine commander, which must have been pretty gritty a lot of the time - how much does she understand of that?

Perhaps this is on his mind at dinner, when she is trying to fill the gap left by his silence?

If he is contemplating how he prefers the simple life in Salzburg to the glitzy life in Vienna, then Elsa has really selected the wrong outfit for dinner!

I imagine him being somewhat flustered by his own lack of clarity. He could command troops without hesitation with enemy ships approaching, but being in love makes everything fuzzy. He can't figure out the moves he needs to make to get to where he wants to be. He knows Elsa is charming, but he knows they aren't supposed to be together. They want different lifestyles and have politics. But, he feels terrible about breaking up with her, even though he knows it's best for both of them. Then, there's the shock of Maria's return. He never envisioned being in this position. How does he get her to stay? He has to at least suspect that she came back because she has feelings for him. How does he figure this out without making a fool of himself? What a mess! And he knows he needs to act soon or she'll be gone again. I'd be downing the wine too..

Ha ha this makes me think that he could actually perform a reprise of “a problem like Maria.“. “When I’m with her I’m confused out of focus and bemused, and I never know exactly where I am.“

… while the nuns are STILL complaining about her when she’s marching up the aisle!!!

Also he knows he might only have a few minutes with her alone, his only opportunity to both find out how she feels and to persuade her not to leave - just yet!

Maybe he is feeling desperate that she may slip away forever this time, and so he has to pin her down as fast as possible, before Elsa sees them talking together.

"Please don't ask me..." is a response which makes her very vulnerable, a bit like "You have helped...more than you will ever know" in The Apology. Both responses, rather than being accusatory like in The Argument, half open a door because they induce a desire to see inside, and to ask what that person means by the comment. Intriguing!

More doors!
I really love this whole discussion. “The reason no longer exists” is SO vulnerable because really what else could the reason be.
I like the idea that on the inside, he is in a rush, aware that he has to get somewhere with her before Elsa intrudes, while on the outside, he starts out so utterly still when he greets her, with that mocking little smile, as though he doesn’t have a care in the world. Then the whole tempo and tone of his manner changes to match the turmoil inside.

That is so smart, indigoblue! I think your explanation makes the most sense.

Golly, I had always been puzzled by why she gazed so deliberately to the left...now I know!

Will have to scamper away, considering what she was thinking then...

All of the above. In a nutshell, I think she is totally confused, and is reenacting that moment when he asks "You are here to stay?", as this must be the most puzzling question of all for her.

-Is he asking her this because he is in a flap about childcare after he gets married because he and Elsa will be so busy?
-Or is it because he is in a tizzy about her leaving before he can declare his love to her?
-Maybe she is wondering whether her response, "Only until another governess arrives" was too much of a put-down for him, and she should have left him more encouragement?

Either way, I'm sure this is why she went to the gazebo to contemplate her future. Maybe she saw Elsa give Georg a kiss on the cheek on the balcony, and decided she had lost out entirely?