"Tintern Abbey" is a nature poem, and nature is always full of transformations: fruit ripens, seasons change...you get the picture. The poem describes the transformation between the young, boyish "William" and the more mature speaker of the poem; it also imagines the future transformation that will change the present Dorothy (Wordsworth's sister) into someone who will have the speaker's deep appreciation for Nature.
The transformation of young William to the more mature speaker is brought about by his recollections of the river Wye in moments of solitary reflection.
Although the speaker seems to include Dorothy in the system of transformation brought about by recollections of nature, he is actually just projecting his own experiences onto her. The real Dorothy is excluded from the system described by the speaker.