Frequent alliteration, combined with steady meter, gives the poem an intense, driving rhythm. Sometimes the alliteration occurs within a single line, as with the /l/, /h/, and /w/ sounds in line 15 and the /d/ sounds in line 16:
Like me who have no love which this wild rain
Has not dissolved except the love of death,
Sometimes it flows from one line into the next, as with the /l/ and /s/ sounds in lines 9-10:
Is dying tonight or lying still awake
Solitary, listening to the rain,
A related effect comes from the simple repetition of certain words, such as "Rain" in line 1:
Rain, midnight rain, nothing but the wild rain [...]
The repetition of consonant sounds (and whole words) evokes the driving, repetitive sound of the rain itself. Alliteration also helps highlight some of the poem's most thematically important words in the poem, such as "solitude" in line 6 ("Since I was born into this solitude"), "love" in line 15 ("Like me who have no love"), and "death" in line 16 ("Has not dissolved except the love of death"). This is a poem about solitude, love (or detachment from love), and the anticipation of death, so it's fitting that the poem's sound effects bring these words to the fore.