The topic of this poem is love. In this poem, the speaker is using the idea of the star that shines bright and eternally as a way of talking about his love for a woman.
In this poem, the speaker talks about how he would like to be as steadfast -- as reliable and eternal -- as a star. But the point of being like that would have nothing to do with the natural world. The reason that he wants to be as eternal and reliable as the star is so that he could experience love that would be that lasting. He wants to stay in love forever and experience forever the feeling of being in the presence of his love.
This is a personal poem in which Keats’s speaker describes the wish to be a “steadfast” lover like the “bright star” which is a distant witness to the earth’s waters and snows (i.e., the changing seasons). The most steadfast star that we know in the northern hemisphere is of course Polaris, or the North Star, and we may presume that this is the star that Keats has in mind for his metaphor of stability and permanence. The topic is the speaker’s love. The comparison of speaker and star is the implicit one that the speaker, being human and therefore changeable, is as impermanent as the waters and snows. The contrast with the star is that the speaker wants to remain close to his love (“pillowed upon my fair love’s ripening breast [line 10])” and not remain aloof in “lone splendor” (line 2).