Break, break, break (1)
The speaker is telling the ocean waves to "break," but we don't realize that until the second line where he finishes the sentence. As we first read the poem, we just see this word "break" repeated three times. It's not a happy word – we break bones and hearts and vases. The word itself is a harsh word, with that "br" sound at the beginning and the hard "k" sound at the end. And the repetition, with the commas in between, "breaks" up the line, suggesting the speaker's "broken" heart…you get the picture.
On thy cold gray stones, O sea! (2)
The repeated long "o" sound (in "cold," "stones," and "O") sound almost like moaning. And "cold" and "gray" surroundings are almost always setting the scene for something sad.
That he shouts with his sister at play! (6)
The children of the fisherman seem unaware of the deep grief of the speaker as they play around happily.