In Arnold's "Dover Beach," the speaker compares the ebb and flow of the tide on Dover Beach to first, man's condition, and, second, the state of mankind's faith. The speaker's sadness is created by his belief that mankind has lost its faith, and he alludes to Sophocles, an ancient Greek philosopher, who, over two thousand years ago, also concluded that mankind was miserable:
Sophocles long ago/Heard it on the Aegean, and it brought/Into his mind the turbid ebb and flow/Of human misery. . . .
And, as the speaker points out, the same misery can be heard "by this distant northern sea," that is, on Dover Beach. The speaker, but alluding to Sophocles, points out that human misery has been relatively constant for thousands of years and in all places. Sophocles sensed human misery while living on the Aegean Sea, and the speaker senses the same misery many years later and in a different part of the world.
In the third stanza, the speaker notes that the "Sea of Faith" once encompassed the entire world "like the folds of a bright girdle furled," but as he listens to the tide now, the speaker hears only
Its melancholy, long, withdrawing roar/Retreating, to the breath/. . .down the vast edges drear/And naked shingles of the world.
In other words, faith, like the tide as it goes out over the shingles on the beach, has gone.
The speaker's sadness over the loss of faith becomes even more clear in the fourth stanza when he says
And we are here as on a darkling plain/Swept with confused alarms of struggle and flight,/Where ignorant armies clash by night.
Unfortunately, the speaker holds out no hope for mankind in this poem: mankind, having no faith or future, is simply a victim of forces in the world that sweep man along. In the world of "Dover Beach," there is no past and no future, only the present, but it is a present over which mankind has no control and no hope of changing.
The only hopeful statement the speaker makes is in the first line of the fourth stanza in which he says "Ah, love, let us be true to one another," but even this statement is quickly followed by the observation that this world "hath really neither joy, nor love, nor light. . . ." In essence, even love cannot conquer the depressing and desperate state of man's hopeless condition.