Agreed. But when the model for critique is the "sandwich" that's what can happen. Because every "negative" is supposed to be balanced by at least one positive and... well, sometimes there aren't that many.
That's not what I understand by the "shit sandwich" technique though, and I've never approached it mathematically with any kind of ratio, never mind 1:1.
I would consider the following to be a "shit sandwich":
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I really like/love (thing that I like) - I want to know more about (something else that I like). There are some things that you need to work on though.
[A few paragraphs dealing with stuff that's not going right - phrased constructively, with suggestions for how to develop/improve the writing etc.]
Overall though, I think this has loads of potential and (something else about it that I like).
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I usually won't end up only saying something positive at the start and the end - I will comment on bits that are working or come across well as I go along. I don't apply any mathematical formula to it, just make comments/suggestions that I hope will be helpful.
The rational behind "shit sandwich" is that if you start and end with something positive it's easier for the person receiving the criticism to see that you appreciate the value in what they're trying to do and your intention is to help. People tend to be more likely to take criticism on board when presented like this - probably because they trust that you're trying to help them, not bash them. Granted they should probably already know that but there's no harm in reinforcing it and like Brigid Barry says above, you don't know the person's current situation, previous experience etc. So I will err on the side of being encouraging rather than discouraging.
I know it sounds like I'm anti-positive feedback, but that's not the case. I just think that critique is hard. And, specifically when we are recommending that folks learn what does or doesn't work in writing by reading and critiquing other works in progress, boiling everything into these "positive/supportive" and "negative/critical" categories doesn't help anyone much. Constructive is going to be more fruitful in the long term.
I think this has become a matter of semantics. A lot of people will consider "constructive" and "positive" to mean the same thing in this context, but you're treating them as separate categories. Also, clearly we don't all have the same definition of "shit sandwich" (or "sandwich technique" or whatever you want to call it).
I agree that forcing yourself to make up (add number here) of "positive" comments, or trying to go for an artificial ratio of "positive" to "negative" is unhelpful. As is trying to individually shit-sandwich every bit of feedback that might be construed as negative. Simply trying to be constructive and helpful - as you say - is better.
I also don't think there's a danger that people will stop trying to improve their writing in response to positive feedback - even insincere positive feedback (the danger with that is they'll decide your opinion's not worth much). However there are a lot of potential dangers in critiques that comes across as wholly negative and discouraging, especially for beginner writers. So I'm going to err on the side of encouraging and make sure all the positive things I say are things that I really mean.