Congratulation?

One of the most pervasive errors in English here is the use of the word congratulations - it keeps being used as congratulation on cakes, cards and in conversation.

This started me thinking, why do we, in the English world, offer each other either congratulations or condolences, plural? Why not a single congratulation or condolence? We can offer words of congratulation and words of condolence, but then the plural rests in the use of words. Does it perhaps rest in the fact, that, to be grammatically correct, we need to append a or the, since these words are, after all, nouns?

So, if I'm not that happy about your achievement, can I offer you a congratulation?