What do you mean, specifically? "Having fun" aside, being emotional about a game is socially harmful/uncool in the same way a precommitment can be.
I'm not sure how to formalize this, but I think you can be emotional in a playful way that sort of transparently emulates real emotional responses. I haven't played many social board games so I can't speak from experience, it's just that imagining the situation my first response is "transparently act betrayed and insulted, so people understand both that you don't mean it seriously and why you don't help them".
The Game of Thrones board game is similar to Diplomacy (so I hear: I've never actually played Diplomacy). You often need to make alliances to survive, but these alliances are weak. It is both expected and required that you will eventually break your alliances, otherwise you will lose. My first time playing this game, I made an alliance with a neighboring House which turned out to be unwise, and severely limited my options. To me, breaking an alliance to win a game (even if it was socially acceptable) didn’t feel right/wasn’t worth the negative feelings, and so I ended up stuck on my island for the whole of the game.
Instead of adapting by learning to be ok breaking alliances, which I considered to be a sub-optimal solution, I fixed the problem by targeting my alliance terms. Now, instead of a general alliance, my offers were along the lines of “I won’t attack you across this border for the next four rounds, if you agree to the same.”
This had the effect of actually strengthening my alliances. Limiting the terms of the alliance to something that could easily be complied with, meant that defecting was no longer expected. The cost goes down, and the benefits go up. In the previous game, both I and my ally would’ve had to leave our mutual border semi-defended, because we knew the alliance wouldn’t hold against a strong enough temptation of conquest. This was facilitated by the fact that the alliance was expected to be eventually broken. In the targeted alliance, I and my ally can leave our mutual border undefended, since we can expect the alliance to hold. This is facilitated by the fact that there would be social sanctions against breaking the alliance. (e.g. I wouldn’t form alliances with that person in future games, because I knew they would break them.)
This lead me to the thought that prohibitions seem to focus on three axes:
General Examples:
It seems to me that making an injunction specific and targeted increases the expectation of compliance. This is important to me, because I seem to dislike injunctions that I am expected to fudge.
Another example of how this plays out in my life: Being enmeshed in a poly network, there is a lot of talking about people (not necessarily in a bad way-- if you ask me about my day though, my answer is going to involve other people). To get around worrying about if I am ever breaking confidence, I specifically tell people I am close to that I don’t consider any information to be private unless it is specifically stated as so (this goes two ways). This way, I get to "gossip" but also people know they can strongly trust me with any information that is prefaced with "This is not for public consumption..." In this example, like the board game, I am turning a broad, vague injuction that isn't strongly expected to be followed ("don't ever talk about other people") into an injuction that can be trusted to be followed by making it specific and targeted.
Relevant to previous discussions on: ask v guess cultures, and the idea that if it's expected that everyone breaks a specific law then the government can arrest anyone they want to