Michael's formulation is perhaps a bit extreme, but I've been known to express similar ideas. In my case, it's mainly in opposition to people who say things like "it's dance music- you've got to play it at dancing speed", or ask why it's just Irish (substitute name of tradition) and why don't we lighten up a bit and do blues* or pop standards, or can't we do Black Velvet Band that dad used to have on a Dubliners LP, or...

So perhaps a better formulation would be something like:

A musicians' session is a conversation between peers who may know each other well, or less well, or have just met for the first time; but who have a common interest and something of a common interest. In the case of old friends, the conversation generally goes along fairly accustomed and familiar lines. With new friends, a lot of time is spent getting to know each other. As in any conversation, shouting, interrupting (except in a friendly and witty way), banging on too long or hogging the limelight, being over- competitive, &c&c are solecisms.

And the conversation isn't totally confined to the players. It's perfectly in order for someone to ask, have you played (say) Madame Bonaparte? And maybe no one has thought of that for a while, so that's added to the fun, especially when someone (usually me) tries to do the arpeggios too fast when they've not played it recently...

*At one festival this year, well fluthered, I ended up playing Irish keyless flute with someone playing blues in a mixed session. It can't have been that bad because he asked me to play with him for his next one. Or maybe he was as drunk as me.