Vancouver

Not really. I'm suggesting people are social animals and will not enjoy you if you piss on small talk. Yeah small talk sort of sucks. Sorry.

I had in mind a lot of other examples to illustrate my point that covered where people are quite likely to want to avoid being explicit in a disagreement besides what I alluded to in the original post but I have a bad enough habit as it is towards being verbose so I left it at a narrower set of circumstances that make the whole post more concise even if overall less comprehensive.

I hoped though that I might have covered myself by starting with the fact that it's heavily contextual which is really more my point. I think in circumstances like those I've suggested in the original post, it sounds like we roughly agree it's unsurprising that disagreement is often either avoided altogether or mooted only obliquely. But then as you point out, there's plenty of other circumstances where it could and really probably should be just fine, but then, as I'm trying to get across, it's very contextual. In the example of the married couple as you say, disagreements of the nature you describe usually are expressed, and it makes sense, the two people involved know each other well, and can adequately sense the stakes involved and comfortably express healthy disagreement. But then your question is more specific, why do people tend to avoid such clearly expressed disagreement when 'out'. I'm taking this to mean, 'in public' so if there's a question of interpretation here hinging on the meaning of 'out' in this context then we could be at cross purposes. I think once again, the complexity of life, what people want out of a given situation, context and tactics play a heavy role. For example the same conversation on the same topic from the same people, like your married couple, could quite understandably play out very differently in public. Again there's so many dynamics at play through which this could be discussed but I have to narrow it to keep my posts at least a little shorter. When this couple are 'out' maybe they've decided to go for coffee, or to visit friends, or they're shopping, or running errands in all of these examples they have purposes to being 'out' and want to get something out of it, they want a pleasant trip to enjoy coffee and company, they want to catch up with friends and hear how they're doing, they want the items they went shopping for or they want the errands completed and not hanging over them. In all such cases it may feel like a distraction from what they're trying to achieve right now to even bother addressing a disagreement, this is especially so if it's very low stakes and not really worth talking about, or if it's extremely high stakes and could lead to an actual confrontation scenario (even in a healthy marriage), so naturally this is something you'll likely try to disengage from if the option to do so is there. So there's this desire to avoid de-railing a situation that is serving a purpose outside of the topic of disagreement and there's also the specifics and dynamics of the relationship and the nature of the topic. Like it or not almost everybody cares what people think, and they care a lot, so while in private a disagreement could be over with "no sweat" people might be concerned that they'll embarrass the person they disagree with in public or give the wrong impression about their relationship, it's likely they needn't have been so concerned especially when the disagreement could be resolved so amicably, but if it's even easier to just stay away from it and discuss it in private then you can bet people would because even if in reality people around them mightn't care, it feels like others are keenly interested and will come away with the wrong impression.

But not to stretch my point too much, all of what I discuss here can be counter exampled, I'm sure you can imagine if the disagreement is just phrased in this or that way nobody could reasonably take offence? Or what if it's not a confrontational discussion and they're people that know each other well, they couldn't ever disagree? I've disagreed with someone in such and such circumstance and it was fine, why can't everyone just do that? The main thing I'm wanting to illustrate is that it's all about complexity and nuance. If you want to know why people will avoid disagreeing in public the reasons are numerous and manifold, you're tired today, you're not interested in the conversation, you're very interested and passionate about the topic but don't want to talk to that person, you're afraid the group dynamics won't support you, you want to discuss it another time, you're clothes are feeling itchy and that's the main thing occupying your mind at this second, you're fairly sure this particular person will take offence whether it's reasonable that they should or not, you don't want to hurt someone's feelings. I guess what I'm saying is I don't think it's surprising or even bad per-se that people won't always reliably be direct and open in their disagreement because a person is aware of so many considerations any given scenario and the dynamics of the relationship with the people involved and will make smart and tactical decisions about when it's actually a good idea to voice explicit and direct disagreement (confrontationally or politely and amicably) this particular time. This is true for almost anything that goes on between humans, an ideology or personal code, like for example disagreement should be expressed openly and respectfully without fear or malice, is a good thing to have and live by but ideals have to contend with reality and to ignore the countless factors that affect how you interact with people and how they interact with you is simply going to end badly, it'd be like trying to have a conversation using some sort of formula or algorithm and it will ring hollow and uncomfortable and being around you will become uncomfortable and a chore. We're a social species that have developed through millions of years of evolution, millennia of culture and years and decades of personal experience many different adaptations to inform our decisions when talking to each other and can't pretend that that doesn't have an enormous impact on what we choose to say.

Because when people have a conversation they try to share common experiences. There are ways to say "I disagree" that won't shut down the conversation. Just saying that leaves the other person no options to continue the conversation because you are shutting down their topic. A few folks here already gave great answers so I won't repeat the advice.