As a native French speaker living in France, I have always been taught to say  VOUS instead of TU to someone I do not know.
However, as I learned 2 other languages I soon realized that this polite behavior does not exist in the other countries. Speaking another language can also be sometimes quite confusing insofar as one language always influences the other. So, I often say TU to a French person I do not know. It is always the same reaction from the other person who considers that we shouldn’t use TU. He or she keeps saying VOUS while I am still saying TU for a few minutes. Then I usually switch over from TU to VOUS or ask the person if he/she does not mind me saying TU provided that this person could become a friend of mine. I would never ask the same question to my employer or another colleague.

The French are not rude people: they say VOUS

Many French people think saying TU to someone you do not know is a rude behavior, especially at work. You should never say TU to your manager or teacher. Although work relationships are changing in the small companies where the managers sometimes accept to be called by their first name, the use of VOUS is not only a matter of polite behavior, it is also a clear-cut difference made between the employer and the employees.

It sometimes happens that the employees call their boss by his/her first name while keeping saying VOUS to him/ her at the same time. A lot of French teachers call their students by their names while saying VOUS to them. The French consider that VOUS is a way of making the difference between the young and the adults. The young should always say VOUS to an adult to show their respect to this person. When I was 18 I remember I was very sensitive to the people who said to me TU or VOUS. I was quite angry when they said TU and I felt offended. I couldn’t bear being still considered as a kid!

The French love respect: they don’t say TU

Using TU can also be seen as a lack of good breeding and respect to the other person. In the French families, a lot of people say VOUS to their stepfather or stepmother. That is why I was quite embarrassed the first time I said TU to my stepmother whereas my sister-in-law, who was 10 years younger than me, would say VOUS.
In the past the wife and husband used to say VOUS to each other. Today, some French people laugh at this old-fashioned polite behavior and they associate it to the old bourgeois families.

In the video which is an extract from the French comedy Le tatoué, the husband played by Louis de Funès is fed up with his wife’s behavoir. So, he asks her to say VOUS instead of TU, a way for him to put some distance from her. Of course, it does not work as his wife laughs at this sudden old-fashioned behavior and it results in a comic scene.
In the suburban ghettoes where the relationships between the young people and the police are often tense, the young can’t stand the policemen saying TU to them. They usually feel offended and can even consider it not only as a lack of respect from the policemen but also as a kind of racism.
However, VOUS can also hurt some French people who think they are rejected. Saying VOUS instead of TU to someone after a relationship has matured could mean you want to keep your distance from this person and that you are not ready to become friend with her/ him.
To sum up, you should always use VOUS at work and with people you have just met, who are older than you or who are only acquaintances. Use TU for your friends, people of the same age as you and for those who have a more matured relationship with you.

tutoyer: say TU

vouvoyer: say VOUS