Tears, broken hearts, sorrow—and that's just the first stanza. Sadness is everywhere in the poem. The speaker was sad when he had to say goodbye to his friend, and he's still sad now, in the present as he writes the poem.
Even though he writes about it, the speaker's sadness is something he keeps from his friends ("in silence I grieve"), for reasons that remain unclear. If he ever sees this girl again he will show her how sad he still is by crying in front of her (and not saying anything).