Elizabeth Browning was being courted by her future husband Robert Browning (also a famous poet) when she wrote this poem.
The tone of the poem reminds me of teenagers in love. More than just infatuation, Browning's hyperbolic use of time and space in the lines
I love thee to the depth and breadth and height
My soul can reach, when feeling out of sight
For the ends of Being and ideal Grace.
demonstrate a forever and always love. These lines cast spiritual shadows on her love for her lover as well.
Her lines incorporating parallelism
I love thee freely, as men might strive for Right;
I love thee purely, as they turn from Praise.
I love thee with the passion put to use
contribute the idea that her love will continue under all positive circumstances.
The tone of this poem is certainly positive, looking to love in the happiest manners. This was indicative of the period, and is certainly indicative of the topic. When people begin a new love, there is little attention to the conflict or strife that might one day interrupt a relationship. The tone demonstrates commitment to love for a long time. The tone is pure and unselfish. The speaker uses many biblical allusions to compare to her love. These biblical concepts of Praise, Right[eousness], and Grace are all beautiful expressions of God's attempt to love mankind and His desire to be loved back. God's love as understood in the bible would be perfect. Thus, she is illustrating how her love for her lover is perfect.