Arnold begins the poem with images of peace and tranquility, but at the end of the first stanza, he mentions the "eternal note of sadness" in the ebb and flow of the tide on the shore, foreshadowing the theme of loss that Arnold discusses in the third stanza. Dover Beach is essentially an elegy on the loss of something fundamentally positive in the world.
The second stanza points up the universality of this loss. Sophocles heard echoes of this loss on the Aegean Sea, and Arnold hears the same "ebb and flow Of human misery" on the shores of the North Sea. This not only links the loss of something important in time but also in place. In other words, loss and human misery haven't changed from place-to-place or from time-to-time.
The elegiac nature of Arnold's poem becomes clear in the final stanza when he discusses the loss of faith in the world. Whereas at one time faith encircled the world--"like a bright girdle furled"--it, just like the tide, is withdrawing from the world and leaving the world "naked." For Arnold, the loss of faith is profoundly negative.