This famous poetic work, as its title makes clear, is all about grief and loss. More specifically, it is about the grief and loss of Tennyson for his friend, Alfred Hallam. It is this loss that permeates the entire poem as the speaker tries somehow to come to terms with his grief and the fact that he will never see his friend again, at least not this side of eternity.
In Section 7, for example, the speaker is reduced to describing himself as a "guilty thing" that creeps around in the early morning, tortured by the fact that he will never be able to see his friend again:
A hand that can be clasp'd no more--
Behold me, for I cannot sleep,
And like a guilty thing I creep
At earliest morning to the door.
In Section 54, the speaker goes even further, after trying to state his hope in his faith, that there will be some afterlife, he clearly recognises his own position of doubt about this question, stating that his friend's death makes him nothing more than an "infant crying in the night" with "no language but a cry." Throughout the poem, the powerful sense of loss and grief permeates.