This poem doesn't just sound like one thing. You hear a whole bunch of sounds going on here. In the first eight lines, the speaker really sounds like the moon's sympathetic pal. Pretend the moon shows up to meet the speaker for dinner, but he looks kind of funny, which elicits this response: "With how sad steps, O Moon, thou climb'st the skies! How silently, and with how wan a face!" (1-2). As he continues to address his buddy, the speaker comes to sound more and more like a close buddy giving him the whole, "look bro, I know how you're feeling. I'm feeling it too. We're in the same boat": "thou feel'st a lover's case. / I read it in thy looks; thy languisht grace / To me that feel the like, thy state descries" (6-8).
While the apostrophes in the first eight lines sound like a convo between two friends, the second half sounds like a guy asking a series of increasingly frustrated, even angry, rhetorical questions:
Is constant love deemed there but want of wit?
Are beauties there as proud as here they be?
Do they above love to be loved, and yet
Those lovers corn who that love doth possess?
Do they call virtue there, ungratefulness? (10-14)
As the final six lines progress, the speaker carefully levels ever more vehement accusations. The implication that Stella is calling his virtuous conduct "ungratefulness" is worse than any of the others. Yikes.
While the poem at first sounds like the consoling words of a sympathetic friend, but then shifts to a series of angry questions, we should note that nobody would ever use sentences as tricky and chock full of internal rhyme as "Do they above love to be loved, and yet / Those lovers scorn, whom love doth possess?" Good luck breaking something like that out in a casual conversation over dinner at Applebee's. Clearly, then, the short O sounds in all the "love" words, as well as "doth" are aimed at getting our attention, at tugging on our mind's ear (if you can picture that).
In addition, we get tons of alliteration in this poem. Just read this section out loud (go on, nobody's looking) and count the number of times your mouth makes the L sound:
Sure, if that long with love-acquainted eyes
Can judge of love, thou feel'st a lover's case;
I read it in thy looks; thy languisht grace
To me that feel the like, thy state descries. (5-8)
It's almost a tongue-twister, isn't it? So what's up with that, and with all the repeated internal rhymes, or assonance? Well, if you think about all the doubling that goes on in this poem (as the speaker continually sees himself and his own situation in the moon), these repeated sounds work as a subtle way to lend emphasis to that aspect of the poem. Even more, we get the picture of a speaker who's sort of running around in circles (in his head, anyway) about his lovesickness. Sheesh, even his sound choices are piling up here. The poem sonically underscores our poor speaker's troubled mindset—a neat effect, even if it's a bummer for ol' Astrophel.