While in France, speak with a low voice. Speaking loudly in public is one of the rudest things one can do in France (unless you are selling fruits on the food market), and logically most French people will not want to help someone if they think she/he is rude. If one can understand your conversation more than two feet away, you are speaking too loudly.
I don't say that all Americans act this way, but this is, by far, the main complaint I hear about American tourists, here in Paris (the second one is that they wear white socks
This is less important in Southeastern France though (Riviera/Provence area), where many locals tend to speak very loudly too. As a general rule, people in Southeastern France are more relaxed (they are a bit like Italians).
We, French people, do smile a lot less, and do not show our enthusiam as much as Americans do. So ... if you find that French people are cold or rude because they do not smile a lot, do not talk a lot, are not enthusiastic about what you are saying, there is nothing personal against you. It is just the way we act, with everybody, because being reserved and discreet is our way to be polite
. Speaking to someone with a broad American grin would be interpreted as insincerity, mockery, a sign of mental impairment or an invitation to have sex.