Assonance
Assonance is the repetition of vowel sounds for a particular poetic effect. The repetition of vowel sounds might contribute to the musical quality of the poem and might also enhance the meaning of an expression.

Assonance is used here to link vowel sounds between lines: the "o" sounds of "love," and "lose" are repeated, forming the pair "love/lose" which at once emphasizes the doomed nature of Narcissus's love and reminds the reader of Echo, who has lost love (and her life).