The eternal note of sadness in.
It seems as if he cannot simply enjoy beauty, peace, and the intimate relationship he apparently enjoys with his paramour. Something always reminds him of the opposite. He is reminded that life was once simple and stable, but modern times have brought doubt and foreboding because of the loss of religious faith.
He sees this loss of religious faith as a process which is only beginning. It will go on and on, with dreadful consequences for humanity. William Butler Yeats expressed a similar foreboding in his poem "The Second Coming" (1919).
The speaker's tone changes in several ways over the course of the first stanza of Matthew Arnold's poem "Dover Beach." The first change comes when the poem shifts from a general third person reflection concerning the scenery in the first five lines to directly address a listener:
Come to the window, sweet is the night-air! ...
Listen! you hear ...
The tone of the beginning of the stanza is calm and peaceful, and uses such words as "tranquil," "calm," and "sweet"; the scene is described as "fair" (meaning, in this context, beautiful).
In the second part of the stanza, the tone is sadder and more conflicted:
... you hear the grating roar
Of pebbles which the waves draw back, and fling,
At their return ...
This "grating roar" is not a peaceful or calm sound; it foreshadows the martial imagery we encounter in the final lines of the poem. Finally, at the end of the stanza, the tone becomes sad or melancholic, and the narrator no longer sees the waves as peaceful, but their sounds repeat:
With tremulous cadence slow, and bring
The eternal note of sadness in ...