Yep, love- that sounds about right. I would say that one of the driving forces or themes behind Browning's sonnet is that love can be redescribed in a variety of ways. Just as one's true love for another is limitless, so are the metaphors or images that help to convey it. In the sonnet, the speaker employs a variety of such descriptions to convey the feelings towards another. There is a powerful notion here that argues that the more one can authentically express one's love to another, the greater that bond is because it has taken a hold in another part of the world. The love conveyed in the sonnet is both of this world and outside it just as it is described in terms that are both spiritual and political. In the redescription of love, a greater understanding of it exists.
Of course, the major theme of this poem is love. The poet is talking about the emotion of love and she is exploring the different aspects of that emotion.
In the poem, she shows love from a variety of perspectives. She talks about love being a quiet, everyday sort of thing. But she also talks about love being a passionate thing.
Anyone who has loved another for a long period of time knows that there are all sorts of different aspects of this emotion. The poet is bringing these aspects out in this poem.
Clearly, Elizabeth Barrett Browning is writing about love. But we sell her poem short if we see this only as an enumeration of the many facets of love. This is actually a very personal statement about the way in which her love is the result of a choice. And it's a statement about the way her love has renewed her. The poet was near 40 years old when she wrote this, a time in life when her culture expected her young passions to have cooled. But she affirms that her love equals anything felt by her younger, idealistic self.
The sonnet is a compact and demanding form. You have very few words to convey your vision. After the opening line, and a general statement about the immensity of her love, she spends more than half of her remaining words testifying to these themes.
"I love thee freely, as men strive for Right"
Do people sometimes love not freely? I would argue that they do, inasmuch as people often form attachments that owe a great deal to mere proximity, familiarity, and need. In many cases, people have little choice about the people they are intimate with (either because they are born into a family or because they fall in with people due to accident, proximity, or social pressures). Human beings often form attachments with these accidental intimates, even though they wouldn't have sought out those people a priori as targets of their love.
It's hard to prove that one's love is "free," in this sense, unless you go out of your way or pay a price for it, as was the case for the poet. As a result of choosing to marry her husband, Robert Browning, Elizabeth was disinherited by her father and rejected by her brother. In this context, the next line seems relevant, too:
"I love thee purely, as they turn from Praise."
She invited scorn by marrying her husband.
And then the poet spends the next three and a half lines juxtaposing her current love against the passions and depth of feeling she experienced in the past:
"I love thee with a passion put to use
In my old griefs, and with my childhood's faith.
I love thee with a love I seemed to lose
With my lost saints…"
Here, she speaks of an intensity of emotion that is frequently assumed to diminish with age. More than that, she is talking about having been disillusioned. She lost faith in certain saints, as sensitive people are liable to do as they mature and observe the evils and shortcomings in the world. But Elizabeth Barrett Browning is suggesting that her love now has restored this intensity. She loves now with as much passion and feeling as she ever felt for anything before, even when she was young and innocent.