If I may add, not only is this spot on analysis, but there's another beautiful layer to this. Because not only do their eating habits reflect how their relationship fulfills them, but it also reflects their choices in regards to each other and what they bring to their relationship.
Ilya meets Shane halfway with Shane's eating (disorder) habits and goes out of his way to find food that can bring Shane pleasure without breaking his rules. Meanwhile, he's unable to find much structure in his own life, and flounders when it comes to healthily processing information. We've seen him lash out, close up, and withhold information time and time again. He's happiest when he makes impulsive decisions that result in the natural introduction of more structure: getting serious with Shane, impulsively adopting a dog (arguably his best decision), and even going to therapy. Ilya is a man who thrives on and attempts to bring joy wherever he goes. This is likely a big part of why he gets along so well with children. Ilya is the little treat—something Shane usually doesn't let himself have. He is indulgence, he is pleasure, and he is living in the moment.
Little treats, friendly reminder, are critical to maintaining mental health. Too little and you have nothing to get you from place to place. Too much and you'll lack the structure to move at all.
Shane, meanwhile, is all structure. And he brings structure to Ilya's life, as well. He comes up with the Ottawa plan; the charity; the shift in publicity that covers their public relationship. But the moment he loses any small amount of that structure, he melts down. Total collapse of the Shane Ecosystem. There's no padding to support between the structure; no alternative way he has to keep afloat. He is the equivalent of a starving man getting injured. Since his needs are being met the the point of having nothing to sustain him, he just doesn't have any of that excess you need to heal. He's all work and no play, and his only escape from the stressors of daily life at this point is through Ilya—through the steps Ilya takes to inject joy into his life. Shane had issues bouncing back from small setbacks. He needs everything to be perfect. But that's unrealistic.
It's also very damaging, long term. Not just for the people doing it, but the people around them.
But, most importantly, it makes him blind to the parts of the plan that are going well on paper, but not in reality.
Ilya can tell when the pleasure is gone, but Shane can't. Shane only has the plan.
Shane can build the skeleton of their lives, and Ilya will do his darndest to meet him halfway, but he has to learn the hard way that excess and joy, things that a skeleton alone does not consist of, are needed to feed flesh and muscle and organs enough not just to be maintained, but to grow.
Alternatively, they are building a life and home around them, and Shane knows how to build the frame, and Ilya knows how to install the insulation, but someone has to set the drywall.
And I would like to suggest that the moment Shane finally learns this is the snickers bar. When he gives in to pleasure. When he lets the unsupported structure he'd built independent of Ilya (career with the Metros) fall like a deck of cards around him.
When he allows himself to seek pleasure and comfort outside of Ilya.
Ilya, meanwhile, learns this bit by bit, but it becomes Real to him in a big way after he talks to his therapist about the fight with Shane about the party invitation.
(I also like how David and Yuna very clearly have a similar relationship, and how Ilya gets along so well with David, who plays the same role he does.)
Okay, I'm done. Thanks for joining me in hell.