Black and white photos should be the standard of photography. When one can't rely on the "crutch of color," emphasis then must be placed on composition, shading, and scope. It is for these three reasons I enjoy this piece. I shall go into detail.
COMPOSITION The Subject matter choice holds the attention. We don't know who this girl is; we're given no context. We see that she is pretty, and someone is handling her face. Is she a model preparing for a shoot? Perhaps she is sad and a lover is consoling her. We just don't know. It is this lingering curiosity that makes one study the piece, and think about it. A good picture ALWAYS make the observer contemplate.
SHADING Even with a good composition, a black and white photo can go horribly wrong if the contrast isn't correct. (I improperly call it "shading" - but you get the drift.) Here, the hand and thumb are the brightest points of the photo. They serve as a beacon - like an arrow, bringing our attention to the subject's slightly parted lips. From there, our eyes are carried along shadows playing across the subject's pale skin cast by her dark hair - away form her face into what is presumably, the sun beyond. The dark, upward arching eyebrow also guides us in that direction, assisting in the visual flow. Together, they help to give the viewer's eyes a natural progression to follow - from-hand to-lips-to-cheek-to-hair - to out of photo.
SCOPE Finally, it is the limited scope of this piece which makes it memorable. Half of the subject's face is hidden. We don't see the face of the person in front of her. We have no knowledge of location. Outside? Inside on a sunny day? Without these clues for our minds to place the photo into a familiar context, it forces us to wonder, to strive to find some rational meaning for the photo, which, will never be provided ( but does it need one? ). For me, when my mind can't find a rational basis, it sets off on wild speculation, which, I happen to enjoy.
WHAT I SEE? So I leave you with this. When I saw this picture my first response was to the romantic; This is the manner in which I've held girlfriends' mouths in the past, directly before or after a kiss to position her head "just so." I speculate that that the person holding her mouth is male, and he cares for her deeply. As such, the photo evokes positive emotions, and I place it in the "Like" column.