For the error bred in the boneOf each woman and each manCraves what it cannot have,Not universal loveBut to be loved alone
These lines really stood out to me because I think what he is saying here is that all people desire to be loved, even by just one person. The "error" here could be that people put up a front and think that if they are loved by many or are popular than that's enough, but the reality is is that men and women just want a personal love.
Time for you and time for me,And time yet for a hundred indecisions,
This stanza reminds me of the passage in Ecclesiastes 3 when Solomon (the writer of Ecclesiastes) talks about how there will be a time for everything... "A time to be born, a time to die, a time to plant, a time to uproot" etc. There is a lot of Biblical referencing in this poem, which makes it seem almost like a prayer of sorts. Even though it is a "love song," I think he is just crying out to whoever would hear him in his time of anguish.
But though I have wept and fasted, wept and prayed,Though I have seen my head (grown slightly bald) brought in upon a platter,
The Biblical parallelism here is referring to John the Baptist's beheading. He is saying that even though he has cried, fasted, and prayed, he will still face death. I read this as somewhat of a lament, because even though he could do the "right" things, death is still inevitable and he is grieving that ahead of time.
The darkness drops again; but now I know
It is interesting to me how Yeats attributes to the Second Coming to something negative and dark. In Christian theology, the Second Coming of Jesus is something that will be a triumphant event and bring hope to the world, rather than darkness and defeat. To me, Yeats is making the assumption that the world is in a bad place which is because of the war, and that the Second Coming might be of something more negative than he thought.