The first of Wordsworth's Lucy poems begins with these lines:
Strange fits of passion have I known,
And I will dare to tell,
But in the lover's ear alone,
What once to me befel.
Even Wordsworth's greatest admirers would have to admit that, of all the Romantics, he is the most likely to write a love poem that is all about himself. In fact, he wrote a sequence of them. The focus of the Lucy poems is always on Wordsworth's own sensibility, his passions and feelings, and the effect Lucy has on them.
This is why Lucy is deliberately portrayed with vague imagery, especially with vague natural imagery. There is some precedent in love poetry for comparing a girl to a flower, but this particularly suits Wordsworth's purposes here, since he also describes flowers in terms of the way they affect his soul. He compares Lucy to a rose, then a violet, and then to a generic "flower." Finally, in death, she is one with the "rocks, and stones, and trees."
By making Lucy such a vague figure, Wordsworth makes her one with all loves. The effect she has on him is part of the way nature in general affects him. However, in writing of her in this way, he also makes himself one with all lovers, since any reader can project his (or, for that matter, her) beloved into Lucy's position in the poems and recognize his or her own feelings.