Why is Jeremiah referred to as the weeping prophet? 

Answer:

"Oh, that my head were waters, and my eyes a fountain of tears, that I might weep day and night for the slain of the daughter of my people! ... I will take up a weeping and wailing for the mountains, and for the dwelling places of the wilderness a lamentation, because they are burned up, so that no one can pass through; nor can men hear the voice of the cattle. Both the birds of the heavens and the beasts have fled; they are gone." (Jeremiah 9:1,10).

Jeremiah is also the writer of Lamentations, which means "weepings."

"For these things I weep; my eye, my eye overflows with water; because the comforter, who should restore my life, is far from me. My children are desolate because the enemy prevailed" (Lamentations 1:16).