In the poem "Dover Beach" by Matthew Arnold, Arnold expresses a rather dreary and pessimistic view of life. He feels that all faith and love, and goodness and peace have departed from the world that he lives in. He paints a picture of his current world as being filled with sadness. However, he doesn't stop at viewing only his world as filled with misery--he also imagines that many like-minded individuals throughout history have had his same perspective. Sophocles, who could see the Aegean Sea, wrote many plays that were very sad, depressing, miserable and frustrating. So, Arnold makes a comparison--he feels that Sophocles, looking at the sea and hearing the waves on the rocks, that it
"brought/Into his mind the turbid ebb and flow/Of human misery."
Since the subject matter of so many of the storylines in the plays of Sophocles was sad and despairing, Arnold makes a connection between how he feels about the world, and how Sophocles must have felt the same way. It's a pretty grand assumption, but one that Arnold does make rather poetically. So when Arnold goes on in the next stanza to state that "The Sea of Faith was once, too, at the full," he must have been referring to a very long time ago, even before Sophocles. It's a rather sad perspective to have of life, and one of the main themes of Arnold's poem. I hope that these thoughts helped a bit; good luck!
Because he heard human misery in the ebb and flow of the Aegean nearly 2000 years earlier.