Consider "When We Two Parted" as a romantic love poem.
This poem seems to have all the ingredients of a romantic love poem, especially because it is so sad and mournful and deals with a love that can never be acknowledged or regained. The tragedy in this poem is that the speaker, in spite of the years since he parted with his beloved, is obviously still very much in love with her. When he heard of her downfall through public disgrace, the speaker says that he shares "in its shame" when he hears her name spoken, even though there is nothing to connect them. Consider the third stanza in connection to this point, which identifies just how deeply the speaker still loves his beloved:
They name thee before me,
A knell to mine ear;
A shudder comes o'er me--
Why wert thou so dear?
They know not I knew thee,
Who knew thee too well:
Long, long shall I rue thee,
Too deeply to tell.
Even though the speaker's love is unknown, he finds that the gossip surrounding his lover comes as a "knell" or a funeral bell to his ear. He finds himself questioning why he feels such a strong connection to his lover and ends up by reflecting that this relationship and his feelings towards his lover will be something he will be struggling with for a long time into the future.
The final stanza talks about how the speaker should greet his love if they were to meet. In spite of all of his feelings, the only option open to him now is to greet her in the words that are repeated in this stanza: "in silence and tears," cementing the tragic nature of a love that cannot be anounced or acknowledged.