Americans are very direct people. When we want something we say "yes" and when we don't we say "no". Furthermore， if we want something different from what is offered， we ask for it.
Here's an example. Suppose I arrive at someone's house and he offers me beer. I don't want beer. Maybe I don't like it or maybe I just don't feel like it. I will say "No， thanks." If everyone around me is drinking something， I would want to be sociable and would therefore ask for something else： "No， thanks. But I'll take a bit of wine （or tea or a coke） if you have some." If I do want beer， I simply say， "Yes， thank you." （If there are only a few guests I might be polite and say， "Please don't trouble yourself." The other person will then say， "It's no trouble at all. Then I would ask directly for what I want.）
Unless they happen to be familiar with Chinese customs， Westerners will not ask you again and again or try to press something on you after you have said you don't want it. In fact， if you say no when you really want something to drink or to eat， you may find yourself very thirsty or hungry.