Why does the speaker keep referring to Dorothy's "wild eyes" (119, 148)? Why are her eyes wild? Is that supposed to be a good thing?

Her wild eyes are a good thing. In the fifth and last stanza, Wordsworth addresses his sister Dorothy, calling her both "Sister" and "dear Friend." Through her eyes, Wordsworth can see the wild vitality he had when he first visited this place, and this image of himself gives him new life. It is apparent at this point in the poem that Wordsworth has been speaking to his sister throughout. Dorothy serves the same role as nature, reminding Wordsworth of what he once was: ...

in thy voice I catch The language of my former heart, and read My former pleasure in the shooting lights Of thy wild eyes. Oh! yet a little while May I behold in thee what I was once, My dear, dear Sister!

Wordsworth then shares his deepest hope: that in the future, the power of nature and the memories of himself will stay with Dorothy. He is implying that he will die before she does (even though she is only a year younger), and hopes that in her memory he will be kept alive:

If solitude, or fear, or pain, or grief, Should be thy portion, with what healing thoughts Of tender joy wilt thou remember me, And these my exhortations!