It's just a cultural tendency around here to be more insular and guarded. This is not to say that Parisians don't ever smile – it's just considered by many locals to be disingenuous to be smiling all the time or toward strangers as we Americans (transplants included) do.
As Travelnut mentioned, the French (at least in Paris) think too much smiling makes one look simple. All Americans do not smile all the time. If you smile at a stranger while shopping they will smile back and so will your server.
While it's fairly common for Americans to smile throughout a conversation, French conversations tend to be more straight-faced. It's not a sign of unfriendliness but simply a different way of conducting interactions.
Everybody gets a smile and it is considered rude not to smile. The smile is free and ubiquitous, like air or sunshine. It's expected and polite. Everyone gets a smile but in France, French people perceive the smile differently to Americans.
It is rude to sit with one's legs spread apart unless in a relaxed context. Rather, sit straight with your legs crossed at the knee or knees together. Feet should not be placed on tables or chairs. If someone is invited to a restaurant or a business function, it is acceptable to arrive at the specific time.
Public displays of affection in France
People kiss acquaintances when they meet them in the street and you even shake hand with the plumber when he comes to fix your leaky pipe: the French are definitely touchy-feely. So, in Paris (and other major cities) anything goes; elsewhere, be more restrained.
French was also the most attractive accent when speaking another language. However, for the French people who were surveyed, the language of love was Italian and the most attractive accent to hear someone speaking French was the English one, taking 39% of the vote.
For Parisians eye contact is a personal matter, much like your choice of wine, or infidelity in relationships. As a general rule, you have to avoid looking strangers in the eye; it's slightly offensive.
La bise is a traditional, common way to greet family, friends, and even coworkers (depending on your workplace). It's the equivalent of both a handshake and a hug. Basically, faire la bise consists of lightly kissing someone on the cheek or cheeks.
It's not normal in French culture to hug, so if you attempt to hug a French person, they may get a little too close, too touchy-feely, might not know what to do with their hands or might just stand there waiting for you to move on to the next unsuspecting Frenchie. It'll make them feel really awkward or embarrassed.
Eye Contact: Direct eye contact is understood as a form of respect. It is considered to be extremely rude not to make and maintain eye contact. Gestures: French people tend to use the 'thumbs up' sign to indicate 'okay'. Making a circle with the thumb and index finger means 'zero' in France.
Avoid asking personal questions relating to one's age, sexual orientation, family or children unless you have a well-established friendship. The French highly value their privacy and the privacy of others. Inquiring about one's salary and finances is taboo.
In 2020, in France, 53 percent of women who were not wearing bras in public spaces were doing it to feel more comfortable. Moreover, 17 percent of them mentioned the wish to free themselves from aesthetic norms imposed on women's bodies as an important reason not to wear a bra.
The main neighborhoods to avoid in the 18th arrondissement are Barbès, Pigalle, and Red Castle. In the morning, a large flea market is set up between Porte de Clignancourt and Saint Ouean, but at night these areas aren't very safe.
Expect Lots of Flirting
But whether you're single or not, in France, there is plenty of flirting regardless of marital status. To both sides, flirting is an art form. So you're going to see it thrown around a lot.
Hugging in France
Unlike Americans, the French do not use hugging as a greeting. Instead, they kiss cheeks (faire la bise) informally and shake hands in formal settings. Because they are not given often, hugs tend to make French people uncomfortable and can easily seem like an invasion of personal space.
Public displays of affection (PDA) aren't a taboo in France. Couples hold hands, hug, and kiss regularly. Perhaps this is the reason why the reputation for French romantic freedom is so widespread.
Oh là là can express anger
(Oh my God, you're starting to annoy me!) Or…just about anything. Remember that, like “Oh my God” or “Wow,” you can use Oh là là to express a reaction to nearly any situation.
un beau brin de fille [ex.]
Oh là là is arguably the most iconic and quintessentially French expression there is. As cliché as it may sound in English, it's actually used regularly in everyday French. This is one of the very few French stereotypes that is actually based in reality.
You will know when a French girl is flirting with you: she'll smile to excess, move her hair a lot, smile at you, blush, laugh hard and loud at your jokes, find occasions to touch your shoulder (or even your knee… oh la la),… and it's not unlikely that she'll make the first move.
Generally the French are a little more closed off and not so open and friendly as other groups, but this doesn't mean that it's impossible to make friends with them. Just remember to go about it in the right way, make an effort to learn the language and culture, and be patient.
The three meals of the day– breakfast, lunch, and dinner– often include both bread and cheese. Maybe that is why the French rarely snack.