Clear eyes, beneath clear brows, gaze out at me,
Clear, true and lovely things therein I see;
Yet mystery, past ev'n naming, takes their place
As mine stay pondering on that much-loved face.
Deep in another's eyes, what do you see? No matter how well known, how loved, there is mystery there, or so says Walter de la Mare. While eyes can be "clear, true, and lovely" they are still another person's, and therefore, unknowable on a certain level. The interior lives of our friends and lovers are forever hidden from us, no matter how close. It's a thrilling mystery, to be sure, and expressed cleanly, clearly, and elegantly, much like the face and eyes in the poem.
As an alternate reading, I think you can also consider this as looking into a mirror. While I don't think it fits fully ("much-loved face" stands out to me as not fitting a mirror reading) it's still workable to address the mystery within ourselves. How long can you view your own eyes in the mirror before they become that of a stranger's?