I just thought about this today. Normally when something good happened to some friend we would say "congratulations" to them but we make it plural, instead of "congratulation". I wonder what is the underlying (grammatical) reason for doing this?
And this got me wondering if there are any other similar situations where when we say a word we would have to pluralize it.
To add to Stangdon's good answer, some grammarians use a more understandable term such as the plural-only nouns (Huddleston & Pullum, 2002).
There are several subcategories of plural-only nouns such as the bipartites (eg. "trousers", "pajamas", "bloomers"), words of compensation and reward (eg. "apologies", "compliments", "regards"1), and formal expressions of feelings (eg. "condolences", "thanks", "congratulations").
1 The singular counterparts of these exist but are generally used in different senses
There is something called a plurale tantum, which is a noun that only ever appears in the plural form.
Some examples are
You can't have "a scissor" or "a trouser" or "a shear" in the normal meaning of these words, and you can't say "His remain was found in the valley" or "He has vast rich" or "She pulled a shenanigan"; you always have to use the plural form.
It's not clear why these pluralia tantum exist. In some cases the item is obviously "paired" or "twinned" in some way (like scissors) but not always (like remains). Congratulations appears to be one of these strange cases.
While the other answers make interesting observations about plural-only or plural-tantum words, 'congratulation' is not one of them. Here is an example:
"Here is my congratulation to you" - reluctantly hands over money for a lost bet
I believe that "congratulations" is now a contraction of typically longer and more formal phrases like:
"I am sending you many congratulations on the birth of your child"
Which can be shortened to:
"Congratulations on the birth of your child"
Other words that work like this are 'thanks':
I would like to thank you by giving you this gift.
Many thanks to you for your help
I am inclined to agree with @josh that many of the examples @user178049 gives are not really plural-only nouns: apology, compliment, condolence, in their plural forms, really do just refer to multiple instances of their singular forms. "Thanks" is a weird one, and also, uniquely amongst the examples we've seen of what @user178049 elegantly summarised as "words of compensation and reward... and formal expressions of feelings", it's not an abstract noun derived from a verb by adding a suffix like -tion, -ence, -ing and then pluralising it.
Lots of interesting points have been raised, but to my mind the answer must be connected to how "congratulations!", "commiserations!", "thanks!", "apologies!" etc are reduced from versions where there's a preceding "many" or "my". Just naming a singular noun as a one-word sentence would feel very weird indeed -- do we ever do this? My gut feeling is that single-word sentences using a plural noun are only valid because we have generalised the rule that you can omit the "many"/"my" etc from the two-word version, but we haven't extended that to singular nouns.
It still leaves me wondering why we use the plural forms in the first place: "please send her my apology", "he sent condolence" etc are grammatical, but they give the impression that a single, specific message is being sent. But when we hear "my apologies", "his condolences", we conventionally understand it to mean the metaphorical extension that it's referring to a sentiment, not a specific message.
The pluralization is from (1) speaking on behalf of many people or (2) in the spectrum of many versions. Example: We send you our congratulations (1 from each); or I send you congratulations (many forms). In many cases the pronoun, subject, and verb is dropped, leaving you with the object in a colloquial, shorthand format.
I think people are overthinking it with plural-only nouns etc.
It is an idiom, used in an exclamatory way, that has been established in the plural. Simple as that.
Etymologically, the phrase means "to share expressions of joy with/for someone". Therefore I assume it would sound weird if only a singular expression of joy was offerred, and therefore the expression got established in the plural.
The word does indeed exist in the singular, and is completely valid when referring to the act itself, as opposed to being used in an exclamatory manner. E.g., see: Self-congratulation.
I hasten to note that it is the same in Greek ( «συγχαρητήρια» ), which translates etymologically exactly the same way. If you were to say it in the singular, people would look at you funny, as if you'd held back in your expressions of joy to only one.
