In this sonnet, Elizabeth Barrett Browning states that the "when" of her loving her beloved, Robert Browning, is day and night, or all the time: "By sun and candle-light."
The "where" of her loving is throughout her entire soul: "To the depth and breadth and height / My soul can reach." It is also a love that goes beyond the earthly: Barrett Browning expects to love her beloved even after death, "if God choose."
The "why" is a little harder to unpack, but it can be found in these lines:
I love thee freely, as men strive for right.
I love thee purely, as they turn from praise.
I love thee with the passion put to use
In my old griefs, and with my childhood’s faith.
Here, Barrett Browning is saying that she loves Robert Browning because, first, it feels like the right thing to do. She loves him "freely," not because anyone has told her she must or because she is needy for love or a man. She also loves him because he is who he is: she loves him for himself, "purely," not because she will be praised for doing so or because other people praise him. She also loves him because she has suffered—because of her "old griefs"—and, finally, because she developed faith in childhood.
The sonnet expresses the all-encompassing nature of the speaker's love. She loves beyond what she can see, and she says she loves with "all my life." This is a transcendent love that includes both body and soul, life and death.
The sonnet also alludes to the book of Romans in the Bible, in which Paul writes in chapter 8, verses 38–39, that
neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.
Browning is taking this expression of the sacred love of God and applying it to her love for a human being.