The reputation precedes the city: Parisians are cold, waiters curt, nobody will help you. Spend a week here greeting people properly and most of that evaporates.
The word is Bonjour — not as a nicety but as a password. You say it on entering a shop, before asking a question, to the driver as you board the bus. Skip it and launch straight into a request, and you have, by the local code, been rude first. The coolness you get back is a response, not an opening position.
Bonjour is not politeness in Paris. It is admitting the other person exists before you need something from them.
Add a Merci on the way out and an au revoir at the door, and the same waiter everyone warned you about often turns out warm, funny, and glad you came.
