And thereupon That beautiful mild woman for whose sake There's many a one shall find out all heartache
Now, the beautiful friend is going to speak. But why does the speaker keep calling her "mild"? Perhaps she is calm, unlike those "noisy" people he referred to earlier.
He says that she is so beautiful that she causes (and will cause in the future) heartbreak for many people. She must be very desirable then.
Notice that "thereupon" and "clergymen" from line 13 don't exactly rhyme? Yeats uses what's called a slant rhyme here, and he does it a couple of times. Check out the "Form and Meter" section, and keep your eyes peeled for more slant rhymes.
On finding that her voice is sweet and low Replied, 'To be born woman is to know— Although they do not talk of it at school— That we must labour to be beautiful.'
Part of what will break so many people's hearts is her voice, he says, which is pleasant. In fact, by characterizing her voice as "sweet," Yeats is characterizing her personality, not just her looks. He wants us to know that she's not just a pretty face.
The woman speaks with her awesome voice, saying that there's something that all women know, and it isn't taught in schools.
So, what is this special knowledge? It takes work to be beautiful.
She's comparing the work poets do within each line, which is supposed to look effortless, with the work women do to look beautiful, which is also supposed to look effortless.
I said, 'It's certain there is no fine thing Since Adam's fall but needs much labouring.
The speaker agrees. There hasn't been anything "fine" since the beginning of time that hasn't required work.
The Adam he references is the first man ever created, according to the Bible. In the story of Adam, he and Eve are expelled from the Garden of Eden, which was a place of peace and beauty. They are expelled for disobeying God. So, maybe having to work to have anything "fine" is part of their (and mankind's) punishment.
The speaker's also saying that poetry is a "fine" and beautiful thing, and thus it takes a lot of work, too.
There have been lovers who thought love should be So much compounded of high courtesy That they would sigh and quote with learned looks Precedents out of beautiful old books;
Once, he says, people who were in love spent a lot of effort reading and reciting quotes from different books.
Presumably, they did this in order to woo the object of their desire. We guess a bouquet of flowers didn't quite cut it.
Even love was one of the "fine" things worth plenty of work. These lovers showed it respect ("high courtesy") by working so hard and becoming educated ("learned") about love. They worked at it.
Yet now it seems an idle trade enough.'
But now, love is something that people consider to be an "idle trade," the same way they consider poetry to be an idle trade.