Why can't the poet say what is on his mind?

In Alfred Lord Tennyson's poem "Break, Break, Break," the speaker laments that he cannot cry out the thoughts inside of him as he addresses the sea in apostrophe. Ironically, though, his poem expresses the very thought that he says he cannot articulate.
In the first stanza, the speaker begins,

Break, break, break,
On thy cold gray stones, O Sea!
And I would that my tongue could utter
The thoughts that arise in me (1–4).

The speaker asks the "Sea" to break on its stones. The speaker uses apostrophe here, a direct poetic address to an inhuman entity, or someone or something that cannot answer. The speaker seems to compare the sea's breaking to his own desire to speak—the speaker wants to take action in the way he wants the sea to.

Next, the speaker continues:


O, well for the fisherman's boy,
That he shouts with his sister at play!
O, well for the sailor lad,
That he sings in his boat on the bay! (5–8)



The speaker wishes "well for the fisherman's boy" and "well for the sailor lad." He wants to articulate these well-wishes to the boy and lad; he wants them to enjoy their lives and feel as well as express joy.

The third stanza reads:



And the stately ships go on
To their haven under the hill;
But O for the touch of a vanish'd hand,
And the sound of a voice that is still! (9–12)



He continues to wish well to the ships, hoping they reach their ports safely. He also wants to wish well for the "vanish'd hand" and "voice that is still." These references suggest that the speaker has lost someone who he wants to wish for but can no longer hope for those wishes to come true.

Finally, the speaker closes the poem with his clearest explanation for why he cannot articulate his thoughts:


Break, break, break
At the foot of thy crags, O Sea!
But the tender grace of a day that is dead
Will never come back to me (13–16).



The speaker repeats his exhortation to the sea, to break its waters on its "crags." Finally, the speaker laments that "a day that is dead" and "will never come back." He mourns the loss of the past in the way he mourns the loss of the "hand" and "voice" in the previous stanza. It becomes clear here that the loss he feels is what prevents him from fully expressing his emotions.