Having been on both ends of the food chain, I will have to agree with PLDM. When my boss took us out to lunch, I was like "ah f-you, thanks for ruining my lunch break". Who wants to spend their lunch hour with their boss? Why should I thank him for that? Now that I have my own little team, I feel guilty when I have to take them out to lunch. I know they would rather be doing their own thing. I don't expect a thank you.
It's a cultural thing I reckon. The boss-employee relationship is different in HK than in the west. Where we come from, the boss wants to be your friend and vice-versa. Not here!