S. R. Faruqi:

In a mirror a form is usually reflected. But it's obvious that it doesn't speak. Thus people suppose the mirror to be amazed/stupefied/perplexed [muta;haiyir]. That is, the mirror, having seen that reflection, has fallen into amazement/stupefaction. From this supposition Mir has created two new themes.

One is that in the mirror the reflection of the face of everyone coming and going comes into view, as if the mirror is staring at the face of every passer-by. The second is that if the mirror is staring at everyone's face, then either it's because it had at some time seen the form of the beloved, and now looks at everyone's face, so that somehow that beloved might again come into view; or else it's because having seen the face of the beloved, the mirror became amazed/stupefied to such an extent that in absolute silence, without a word, it keeps staring at everyone's faces. (In the state of stupefaction this often happens.)

If we assume the mirror to be the heart, then we can say that the heart had at some time seen the reflection of the True [divine] Beloved; since then, it has been in stupefaction, and stares at the face of everyone coming and going. In one small verse, to comprise so many meanings-- if it's not a miracle, then what else is it?

FWP:

SETS
MOTIFS == GAZE; MIRROR
NAMES
TERMS

The first line doesn't specify a subject; we can only tell that it's a third person singular one. This makes us curious, and under mushairah performance conditions we're obliged to wait to have our curiosity satisfied. Then, of course, in the second line it's not really satisfied at all. The line simply asks the same question(s) that we ourselves would ask. But the complex possibilities for answering the question don't alter the (faux-naif) effect of simplicity, or the mood of amazement that the verse creates.