Bright Star, would I were stedfast as thou art— (line 1)
Because this is the first line of the poem, it is extremely important in terms of setting up the major themes in what will follow. In this line, we hear a man addressing a star, the North Star, and saying how he wants to be like it. Now, there are many ways one could think of wanting to be like a star. One person, for example, might think, "Gee, I really wish I could send out light that would travel millions of light-years through space." Another might say, "Wow, I really would love to be a giant ball of flaming gas." But does Keats's speaker want to be a star for these reasons? No: he wants to be "stedfast" as the star is. This shows us the premium he places on loyalty.
And watching, with eternal lids apart, Like nature's patient, sleepless Eremite (lines 3-4)
The images of loyalty are still in the forefront as the speaker's description continues. Now, it becomes clear that it is loyalty to something. How do we know this? Because the speaker tells us that the star is "watching" something. Also, the word "patient" in the simile of the "Eremite" might call to mind the patience of someone who cares for others. The only thing is, we still don't know what the star is watching, or who or what (if anything) it is caring for.
The moving waters at their priestlike task Of pure ablution round earth's human shores (lines 5-6)
Now we get an answer to those questions, but these answers only raise more questions. What the steadfast star seems to be spending its loyal attention on is watching the waters of the earth flow around it and purify its "human shores." So…what are the "moving waters" loyal to? Are they loyal to the humans or are they loyal to the shores, to the earth itself? It kind of sounds like they are washing away the generations of humanity, doesn't it? So, that means the star has loyalty for the waters, and the waters have loyalty for the earth and its barren shores…but nobody seems to have much care for humanity. Once the description of what the star does has come to this point, do we really expect Keats's speaker to feel loyalty towards the star?
No—yet still stedfast, still unchangeable, Pillow'd upon my fair love's ripening breast (lines 9-10)
Well, now we get our answer: "No." Keats's speaker no longer feels loyalty for the star – or, at least, not totally. He wants to take the same eternal attention that the Star expends on scenes of cold, empty, barrenness, and transform them into loyalty for a human object.
To feel for ever its soft swell and fall, Awake for ever in a sweet unrest (lines 11-12)
The repetition of the words "for ever" here seem to act out the eternity of the speaker's imagined loyalty for his girlfriend. The tenderness of the description, with its soft s and f sounds, and general warm and fuzzy feeling, helps explain why he would have such strong feelings of loyalty in the first place.
Still, still to hear her tender-taken breath, And so live ever—or else swoon to death— (lines 13-14)
The beginning of the sonnet's couplet evokes some of the same feelings as the two lines that precede it. Here, once again, repetition is used to convey the eternity of the loyalty the speaker desires, and it also gives a sense of the emotional intensity of his desires. And yet, we get something new here too, something extreme. What is this? Well, if someone told you that they'd prefer to die if they couldn't spend the rest of eternity with their head resting on your chest, wouldn't you consider that a pretty extreme form of loyalty? It makes sense that this is the last line of the poem – you can't really take the theme any further from here. (That doesn't mean you shouldn't think about how to do it – if you come up with anything, why don't you try to write your own poem or song or whatever about it?)