With what topic is the speaker concerned in this sonnet?

"Bright Star" is widely considered to be the last poem John Keats wrote before he died at the age of twenty-five. Scholars believe the "bright star" addressed in the poem to be a representation of Keats's great love, Fanny Brawne. 
The poem is a love poem and describes the speaker's wish to be as steadfast and as constant as a star in the life of his love. That being the case, there are really two main topics with which Keats is concerned: in literal terms, the poem is entirely addressed to a bright star, possibly Polaris, and its "splendour hung aloft the night," "still steadfast, still unchangeable." On a deeper level, however, the sonnet addresses the topic of love: the speaker yearns to stay "pillow'd upon my fair love's ripening breast/To feel forever its soft fall and swell." The bittersweet final lines of the sonnet—"Still, still to hear her tender-taken breath/And so live ever—or else swoon to death"—give an implication of why the speaker so yearns to possess the steadfast and permanent qualities of the star he so admires. The speaker would love to stay forever pressed against his love, but, the wistful tone implies, he may be prevented from doing so by a death that may come too soon.
The star described in the sonnet is portrayed as a "watching" figure, always alert, always at its "priestlike task" of keeping an eye fixed upon its charge, the Earth. We may infer from the poet's wistful tone that he is particularly loath to depart his position upon his love's "ripening breast" because he feels he is responsible for keeping watch over her and does not want to be denied the opportunity to do this forever.
https://www.poetryfoundation.org/poems/44468/bright-star-would-i-were-stedfast-as-thou-art