Help concerning Chinese culture--?

I am currently seeing a chinese doctor, he has lived in america for about seven years... I'm only 21, but have been sick for a long time and he is a very skillful chinese qi gong practitioner and has been helping me. He very kindly offered to have his wife teach me how to cook so that I can eat well, in exchange for practicing english with her, which she doesn' speak terribly well. They aren't very wealthy and live in a small apartment, and yet every time I go over there his wife sends all the food we make home with me. I did offer to pay for it, but she told me not to. Sometimes I feel bad about it, but I really love going over there and want to learn from them. Can someone tell me about chinese culture and relationships in a situation like this? I basically just need some advice, I want to do something for them, and am also a little unsure of how to treat the situation, especially since they keep inviting me over and are so kind to me. I really appreciate it!