grammar - Help on a specific usage of こそ

好きこそものの上手なれ。 We tend to be good at those things we like. / People become best at what they love the most.

How does the こそ work here? Is this use of こそ common outside of proverbs?

Answer

Edit in revision 4: If you are interested in this proverb, do not miss the answer by Matt to another question, where he cites an earlier form of the proverb.

Edit in revision 2: I rewrote the answer completely to give a more detailed explanation. If you prefer a shorter answer, see the old revision.

こそ signifies emphasis. In other words, it adds the meaning of exclusiveness (as sawa wrote in a comment to this answer). It is not archaic, and it is common to see こそ outside of proverbs or set phrases (see Paul Richter’s answer for examples). In this answer, I will try to explain 好きこそものの上手なれ, focusing on the role of こそ.

Without こそ, the sentence would be

好きはものの上手なり。 Liking makes mastery.

好き means the action of liking something, just as in modern Japanese. ものの[上手]{じょうず} is an archaic phrase which means a person who is very skilled at something. なり is a copula in classical Japanese. Therefore the literal meaning of the sentence above is roughly “Liking = A very skilled person,” but obviously this does not make sense because an action is not a person. I think that it means “Liking makes a very skilled person,” or in short, “Liking makes mastery,” but I do not know the exact reason this “action = person” construct can be used.

Now we add こそ and the sentence becomes

好きこそものの上手なれ。 It is liking that makes mastery.

好きこそ means “liking, and nothing else.” So the sentence means that liking and not anything else (such as aptitude) makes mastery. A similar meaning can be expressed in English using a cleft sentence “It is … that …” like the sentence above.

I changed the auxiliary verb なり at the end of sentence to なれ when I added こそ. As Chocolate pointed out in a comment, this is an instance of the grammatical phenomenon called 係り結び. In classical Japanese, when a phrase/clause ending with particle こそ modifies a conjugating word at the end of a sentence, that conjugating word takes 已然形 instead of 終止形. Similarly, when a phrase/clause ending with one of particles ぞ, なむ, や, and か modifies a conjugating word at the end of a sentence, that conjugating word takes 連体形. This phenomenon is called 係り結び.