In this famous reflection on the delights of nature and how nature is capable of being the balm to our exhausted beings, Wordsworth focuses on his sister Dorothy, who is with him on his walk as he views the beauties of the countryside before him. Note the answer to your question that the poet gives:
My dear, dear Friend; and in thy voice I catch
The language of my former heart, and read
My former pleasures in the shooting lights
Of thy wild eyes. Oh! yet a little while
May I behold in thee what I was once...
Thus we can see that looking at his sister and her "wild eyes" makes him remember the way that he was before in his passionate youth and how he regarded nature then, which is something that he had already covered before earlier on in the poem. Regarding his sister thus helps him to recall how he was once, which emphasises how much he has changed in the interim.