Why does matthew arnold mention sophocles in this excerpt from "dover beach"

the purpose of introducing Sophocles into the poem is to show that even long ago, long before Arnold even began to write poems, people saw that human misery flows through the body like a rising and falling tide. then he goes on to say that even now we have acquainted ourselves with those same thoughts. take a look at the language used in the poem, as well. "the eternal note of sadness"... "human misery"... i'm getting at one of the answers here, but i want to see if you're on the same page with me.