This 'Thank you' business is plain daft. Does anyone know its history?
I'm familiar with the convention, but have offended against it more than once, just out of the habit of thanking my partner for the pleasure of a dance. How, on earth, did words that in any other setting on earth would express gratitude or appreciation come to mean 'This is so awful, that I'm going to stop now and return to my seat'?
It's an "honorable" nuance of the Latin culture, and it's based on giving
respect.
In AT, a "Thank you" is given during a tanda to end it when anyone of the partners want to end it prematurely, for whatever reason, good, bad, or otherwise.
Most of the time, it's trying to be polite to someone you no longer want to dance with before the end of the tanda. It's kinder than saying "'You suck,' 'I've had enough of you,' 'You're a horrible dancer,' 'You're getting an erection-EWE,' 'You're creepy,' 'You are an arrogant ****,' 'You talk too much,' 'Stop teaching,' 'Your breath stinks,' 'You've got bad BO,' 'You marinate too much cologne for a guy,' 'Your abundant dousing of perfume makes my eyes water,' etc, etc, etc." Sometimes, there are physical reasons other than that (e.g. cramps, feet hurt, injury acting up, shoe breaks, wardrobe malfunctions, etc.)
It allows the "Thank you-er" to give the "Thank you-ee" an opportunity to save face and preserve their pride. It gives the "Thank you-ee" an opportunity for the "Thank you-er" a way to un-offensively end a tanda early via a commonly agreed to signal, as it is uncomfortable for people to be brutally honest a lot of times.
Whatever the reason, it is the norm and as all of us try to adapt to the rules of the particular culture we want to exist in. This is just one of those things particular to the Tango culture that make it what it is. It's considered to be good manners.