Here's why:
* Knell: A knell is the sound of a bell, typically rung to announce a death. This immediately evokes a sense of loss, sadness, and finality. The speaker feels that the parting is like a death.
* Mine ear: This emphasizes the personal and emotional impact of the parting. The sound of the knell is not just a distant sound but is directly felt by the speaker, signifying the deep and personal nature of the grief.
Overall, the line "a knell to mine ear" establishes a tone of intense sadness, grief, and finality. It sets the stage for a poem filled with despair over a lost love.
The poem's speaker is not just mourning a separation but experiencing a sense of complete and irreversible loss, echoing the finality of a death.