Let us remember the central image that forms such a major part of this excellent poem. The conceit, or surprising figure of speech that is renowned in this poem is the comparison of the speaker in this poem and his beloved to the two feet of a compass. Note the way that this conceit is presented and how it presents the speaker and his beloved:

If they be two, they are two so
As stiff twin compasses are two,
They soul the fixed foot, makes no show
To move, but doth, if th'other do.
And though it in the centre sit,
Yet when the other far doth roam,
It leans, and hearkens after it,
And grows erect, as that comes home.

Note the way that this image allows the speaker to present a mystical kind of union that exists between himself and his beloved, a union that even death itself cannot break. It also presents their reunion as being an inevitable process, as both remain connected and together, and just await that moment when they can be together again. The speaker therefore has no fears about bidding farewell, because he knows that really it is not a farwell: the binds that tie him to his beloved are so strong that even death cannot sever them, and their reunion will be just a matter of time.