Or gazing on the new soft-fallen masque Of snow upon the mountains and the moors— (lines 7-8)
The same goes for this quotation as for the last one. Here, though, the point really gets driven home. The idea of a cold, barren, blank mask of snow landing on top of the cold, barren, blank mountains and moors with no sign of civilization in sight just gives us a chilly feeling. It's no wonder that this image of supreme isolation provokes the outburst that begins line 9.
No—yet still stedfast, still unchangeable, Pillow'd upon my fair love's ripening breast (lines 9-10)
Here we see the speaker's rejection of isolation – he would rather spend his time in close connection with somebody else, not "in lone splendor hung aloft the night." Of course, you could say that he just prefers a different kind of isolation – the isolation that lets him and his girlfriend spend time together and away from everyone else. But the speaker of this poem would probably say that there's a world of difference between the two. What's your take?
And so live ever—or else swoon to death— (line 14)
Here we see the final expression of the speaker's thoughts on isolation. He values human contact so much that if he can't live forever with his head on his girlfriend's chest, he would rather die. He clearly views the isolation of the star as so horrible that he would choose complete non-existence simply to avoid it. Of course, you might wonder whether there might be some middle ground between these two options…