Strong photographic composition is the reason some images jump out to us; our eye keeps moving around the frame, and all the elements just feel "right".
We want to keep looking: But why is that? Here's a breakdown of some of the strong compositional elements in the above photo (one of my favorites from Sam + Sal's Surprise Wedding Ceremony).
Even though the main subject is centered in the frame, notice how many elements intersect with the THIRDS: her eye is exactly on the upper THIRD line; her elbow perfectly hits the lower right THIRDS intersection; even the background elements like onlookers and the trim on the walls plays nicely with the THIRDS.
But most importantly: notice how the connected arms perfectly follow the lower THIRD line, leading your eye from where it "enters" the frame (usually from the LEFT in Western cultures), across and up in a strong triangle to the subject's face, and back out of the frame on the right...with a lovely serendipitous pointed pinky finger that leads you back to the subject's face (even the tiny gesture of a finger can make or break a composition!)
Without getting TOO technical, contrapposto is a technique from art history where the figure's legs, hips, torso, and head achieve a pleasing dynamic balance when aligned on a different diagonal planes, and move in opposition to one another. Basically, it's the opposite of standing straight forward with weight evenly on both feet. Imagine a stiff posture like a Gingerbread man cookie, compared with the leaning, sensual posture of Michelangelo's David (the epitome of contrapposto).
In photography, diagonal lines are powerful, and leading lines keep bringing our eye back to the subject.
Okay, cool. But...are you really thinking about all of these elements every time you press the shutter?
Yes and No.
Years of practice and study have given me a deep understanding of the visual language of photography. While shooting, I'm constantly making a million small decisions about camera settings, where to stand, when to press the shutter, what the light is doing, what the humans are doing, what's about to happen, etc. With every shot, I'm trying to create the strongest possible image.
Often I am "lucky", but not in the sense of a "lucky shot" like handing a phone camera to a baby and letting them shoot 1,000 images until maybe 1 is good. When photographers speak of "luck", it's more like the rewards that come from years of training to be READY for that luck: the perfect light, the candid expression on a subject's face, the divine relationship of elements in a photo that you don't notice until you return to the image later.
Like that pointed pinky finger; it just kills me.