I just cannot hold myself but to reply!

You have to realize that your boss bought lunch for u is kind of his "right", or in other words showing his power. You are not in the position to buy lunch for him, because you r not "high" enough. You should let your boss enjoy his power by appreciating he bought lunch for you, but rather than showing him that "who cares about the free lunch, I can pay you back." In simple words, you will have to know your position, and behave accordingly. When you become a boss of somebody, you can also buy lunch for the others, in a way showing your power.

Remember, you will need to know, to understand who u r! I remember that I brought our group for lunch, a guy who just came from China, insisted to pay for the whole group (I should pay). Honestly, he really pissed me off!! I do not like him since then.