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.