"Exult O shores, and ring, O bells." When and why does the speaker say this in "O Captain! My Captain!"?

This line appears in the last quatrain of the poem—in the fourth line from the end:


Exult O shores, and ring O bells!
But I with mournful tread,
Walk the deck my Captain lies,
Fallen cold and dead.


The speaker says the shores, by which he means the crowds on the shore, should exult or celebrate and ring the church bells because the ship has won an important battle.

The speaker is contrasting the joy he feels—and knows the people on shore feel—about the victory with the great grief he feels that the battle has cost the captain his life. The speaker is caught in an emotional turmoil, torn between his joy at the victory and his distress over the death of his leader. The ecstasy the narrator experiences, emphasized with the exclamation point and the repeated exclamatory "O," is juxtaposed against the shock he carries in his body that leads him walk with a "mournful tread."