and the portrait show seems to have no faces in it at all, just paint
From statues, the speaker turns to portraits. Remember that a portrait is a just a painting of someone's picture. It's like history's Instagram. Only, for our speaker, a show of these kind of paintings is just not doing it for him. He doesn't even recognize the people in the portraits. To him, it's "just paint."
So, now, this love-in has ruined two types of art for our speaker: statues and portraits. Statues just don't stack up to the flow he feels from love, and now portraits seem even worse. The speaker keeps comparing his love to art, and art keeps coming up the big loser. It just can't quite capture the awesomeness that is real life.
you suddenly wonder why in the world anyone ever did them
The speaker is so put off by the portraits that they seem utterly meaningless. Wait a sec. Did we say "speaker"?
Hmm. Here the line tells us that "you suddenly wonder." It's not "I suddenly wonder" (our emphasis). It's as if the speaker and the addressee's drifting back and forth is now affecting the poem.
Here the speaker records not his own thought, but the thought of the "you" in this poem. Is this the speaker thinking for you? Or does it instead show just how in-tune our speaker is with you? Where does one stop and other begin? It's like he can read your mind. Yikes.