I think that the most elemental answer to the question is that Wordsworth is able to express his past experiences in childhood in such a manner that his poetic expression almost helps to give words to our own experiences. Certainly, there is much to be said about how Wordsworth's experiences in childhood are similar to our own. Yet, the reality is that his ability to lyrically express these childhood experiences is what makes Wordsworth so special. Wordsworth is using his own childhood experiences to connect with our own. One of the fundamental premises of Romanticism is the idea of being able to externalize the internal. In universalizing his subjective experience, Wordsworth is able to fulfill one of the fundamental tenets of Romanticism. The subjective experience being poetically experienced so that more can share in it is one of the basis of the Romantic movement. Wordsworth's own imagination of his childhood is meant to help validate our own experience. I don't think that he is writing to make his childhood reveries more significant than our own. Rather, he is poetically expressing his childhood experiences to a point that enables us to do the same. It is this universality and connection that makes Wordsworth's expression so powerful. To be able to reflect on our own childhood and find that Wordsworth experiences the same is where there is validation of the Romanticism thematic approach. At the same time, doing so is what enables us to believe that our childhood holds a magical and poetic quality to it. In this, one sees why Wordsworth's childhood and our own similarity to it holds thematic meaning.