Why is the plural 's' in French always silent?

It’s just…silent.

Many final consonants in French are silent, with some exceptions (c and l, for example, are usually pronounced). You’ll hear final consonants pronounced when the word is followed by another that begins with a vowel, e.g. deux as opposed to deux hommes.

As to the “why”, I dunno. It just happened somewhere along the way in the development of the language.

I’m not French (and I doubt there are many of them on an English website), but I’ve got the feeling that they’re not completely silent – i.e. if a French person just said “livres” without context, another French person would hear it’s not “livre”.

It is just the French pronounciation. It may be weird to us for it to be silent, but it is weird to them to pronounce the “s.” Many things at the ends of words are silent. In the simple phrase “ils jouent” (they play) the final -ent is silent, so the phrase is pronounced sort of like “eels joo.”

@Vincentt no, there is no difference in pronunciation between “livre” and “livres” in isolation.

Final consonants were deleted before words beginning with a consonant. Some sources say this happened c1500, but I don’t know much about the history of French. But this means that in modern French, the “s” in “mes” is silent in “mes livres”, but it is pronounced in “mes amis”.

As for why this happened… who can say why any pronunciation changes happen.

I’m not a native French speaker, nor am I as well-acquainted with the language’s history as I should be, but one of my French professors once said that during the linguistic revamp the courts wanted to make their country’s language “elegant and musical”. This leads me to believe that the silent end consonants are a purely aesthetic choice for euphonious purposes. If we pronounce the zeds in “mes livres” it sounds a bit clunkier than the silent /s/.

@morphail is right. French is my second language (and, by the way, we do have a couple of Quebecer Jellies who probably spoke French before English) and I can tell you that there really is no difference when you put livres in isolation.

@muppetish I doubt very much that French courts in the 1500s had enough power to force every speaker to change the pronunciation of every word.

The silencing of the plural /s/ is very old, dating from at least the end of the use of cases in old french, if not before. This followed a trend among Romance languages, going all the way back to the last years of spoken classical Latin, of de-emphasizing the final consonants of the last syllable of the phonic group. That trend just went further in the Gallo-roman empire than elsewhere.

I find it useful, when pronouncing French words, to aim for those final letters with the shape of the mouth, and I get a subtly more correct pronunciation.