There's a reason they call it the L-word. At least, that seems to be the driving notion behind Wordsworth's use of sound in "Strange fits of passion." L-words, and L-sounds in general, seem to be scattered throughout the poem, and it's not just about love. Let's leap in for a little look.
We first see the L sound used in the poem's initial stanza:
And I will dare to tell,
But in the Lover's ear alone,
What once to me befell. (2-4)
We have "will," "tell," "Lover's," "alone," and "befell"—that's some serious consonance going on in just three short lines. We also get L sounds coming to us via alliteration in line 5: "When she I loved looked every day."
The final stanza, too, lobs loads of L's at our locales:
What fond and wayward thoughts will slide
Into a Lover's head!
"O mercy!" to myself I cried,
"If Lucy should be dead!" (25-28)
Here we get "will slide," "Lover's," "myself," and "Lucy"—leaving us with lots more L's to linger over as we leave the last lines.
Of course, the question remains: what gives with all the L's, Wordsworth? Sadly, he's not here to answer our question, so we're left to make connections between Lucy's name (which starts with L) and the other L sounds of the poem. It's as though the speaker's love for her influences his choice of words.
And of course, this is a poem about that biggest L-word of them all: love. When we consider that this is about one man's love of Lucy, then, we can see all these L's as subtly emphasizing the poem's thematic focus. Isn't that just…lovely?