To feel for ever its soft swell and fall (line 11)
This line brings out even more the sensual, physical aspect of the poet's feelings of love. (Hey, he's even got the word "feel" in there.) Also, the repetition of those soft s and f sounds creates a general atmosphere of softness and gentleness that goes with the physical picture he is describing. (We know, this sounds kind of weird, but just say the poem out loud and we think you'll know what we mean.) Now, some people might think it's a little weird – even possibly offensive – that the speaker spends all this time talking about "it" (i.e., his girlfriend's breast) without even mentioning "her" until line 13. But we don't think this is an accurate reflection of what's going on here. Remember that what he's really talking about feeling its "soft swell and fall" – so that feeling the breast move actually shows the living, breathing person underneath. This becomes especially clear in the second-to-last line of the poem. This emphasis on the physical aspect of the person might be designed to make the brevity of human life all the more poignant. What do you think?
Awake for ever in a sweet unrest (line 12)
Here, we get an even clearer picture of the speaker's devotion to the woman he loves. Instead of wanting to fall asleep with his head "Pillow'd" on her breast, he wants its movement to keep him awake forever – could this be so he can consciously savor the moment forever? That said, another possibility suggested by that word "unrest" is that the speaker is, you know, getting all "hot and bothered" by having his head on his girlfriend's chest for all eternity.
Still, still to hear her tender-taken breath, And so live ever—or else swoon to death— (lines 13-14)
These lines bring the poem – and its discussion of the theme of love – to a close. The speaker says that he would like to live forever with his head motionless on his girlfriend's chest ("forever" and "motionless" capture the double meaning of the word "still" in line 13), and if he can't do that, he would prefer to swoon to death. This shows that, for the speaker at least, at least at this moment in his life, love and physical contact are the most important things on earth.