It' still feels kind of odd to me that people in my boyfriend's family just don't say "thank you". It's just not a habit that they have, they don't thank each other for small favours or for doing things they already agreed to do.
When we first started dating, my boyfriend would often answer "why?" or "for what?" when I thanked him for something nice that he did, and I was just as confused by his reaction as he had been with me thanking him for something as little as making me a cup of coffee, every single time he did that, even though he did so every morning. At first I thought it was just a him-thing, but once I got to know his family better, I noticed that it really just isn't their custom.
It took me a bit longer to understand that saying thank you for something is something they'll do for unexpected things, like an unprompted gift that someone got you because they saw something little at the market that they thought that you'd like, or running an errand for you that you had been procrastinating but never asked them to do. When someone goes out of their way to do something that they had no obligation at all to do for you.
And they don't see helping family as an optional favour, it's just what you do, just like eating from plates and wearing shoes on your feet. Being thanked for something so obvious would be downright insulting, like someone thanking you for flushing the toilet or your boss thanking you for showing up for work sober when you've never given indication that you might ever do otherwise.
For them, being helped by family, and helping them in return, isn't a rare treat or special privilege, but a beyond obvious fact of life.
