Since so many good comments have been written here, mostly on the psychological side of the characters, and they are all excellent, I decided to comment upon a very present entity and that is WINE.
Notice that, until the last scene, everybody drinks white, mostly CHABLIS, an acid one. But on that last scene Resnais shifts to RED. It is no accident, it has in my modest opinion, a way that illustrates a very fundamental change in the feelings that occurred in that lunch.
Criticism and over-analysis, ever present till that event, give way to peaceful acceptance of the characters by the father, Without hypocrite sensibility, that he refuses, but with warmth and tolerance.
Well, I do believe, by some 55 years of experience, that white wine (dry, European style) makes one restless and sometimes bitter.
Red wine makes one more relaxed and happy.
I do not know which kind of wine Resnais prefers, but since he is a Breton I would not be surprised, that it is WHITE. Maybe that is the reason why His movies are so difficult to decode. They are also some of the most magnificent works of cinematic art..
