Outsiders are often surprised to hear North Africans from the Maghreb speaking to each other in French, rather than in Arabic. There are several reasons for this.
One reason, explained to me by native-speakers, is that among the educated, French is considered a higher-class language than the local varieties of Arabic. In some cases, people want to demonstrate to others, by speaking French, that they themselves are well-educated and of a high-class.
This situation came about because most uneducated speakers do not understand Modern Standard Arabic (also known as Classical Arabic). Therefore, the local dialects of Arabic are reserved for speaking down to the lower classes (maids, guardians, storekeepers, or casual conversations).
Another reason, explained to me by native-speakers, is that local varieties of Arabic tend to be quite poor in vocabulary when compared with French. For example, I have often heard children who speak English in North Africa make a statement such as, “My foot hurts.” What they mean is their knee hurts, or their upper leg hurts. The reason they are using the word “foot” in English is that they are thinking in the local variety of Arabic and translating into English. In the local Arabic dialect, anything below the hip is called “foot.” This is only one example of how the local Arabic is extremely imprecise in terms of vocabulary and communication.
Precise words DO exist in Classical Arabic (the version of Arabic used in the Koran, in print, and on television newscasts). However, according to native speakers, if someone were to know the precise words and use them, those words are not generally known in the North African region. Therefore, the Classical Arabic tends to be less useful than French (in general conversation) in the Maghreb regions of North Africa.
A third reason, also explained to me by native speakers, why two people from the Arabic countries of the Maghreb might be speaking in French is that their original relationship actually developed in French. Many times people from North America are not aware that our relationships with people actually develop in a language. Change the language between friends or family, and you actually change the relationship.
When I first moved to the region, I had heard that many in the upper classes often speak French to each other, but I didn’t really believe it. One day I went to a party, and I heard the husband and wife speaking together in French, even though they were North African. I asked about it politely, and they told me that they had both met as students while attending the same French school. Therefore, their relationship developed in French, and French then became their family language.