A Guide to Cropping Through the Lens
Above are Photoshop's built in cropping overlays. In the top left is the golden ratio, which as you can see, falls almost perfectly against the right side of the image, on the models cheek. Meaning, according to the human eye (and math) there is a perfect amount of distance between the right edge of the image, and the model's jaw. Similarly, there is a perfectly sized triangle in the second image, stretch from corner to corner, and coming to a point at exactly where the content ends.
The bottom images are a little more simple. The left being the rule of thirds option. The whitespace on the left of the image, along with the fading of the lighting, both end at the first vertical crop line. Her eye falls directly onto the crosshairs of the first quadrant, meaning it's likely the first thing you'll notice about the image. On the right, the background meets her feet at almost exactly the third vertical crop line.
Most importantly though, is this one. The golden spiral. In this particular image, you can see that the spiral fits perfectly into the empty space of the image, and wraps in sync with the shape of the model's outline. This leaves the viewer's eye needing more satisfaction, and they'll keep reviewing the image until they get it.
These are some terrible crops of the same image. In the third one, if we tried to center the image, the rim light in the background offsets it entirely, telling us the image cannot be centered. The first is cut far to much to the left and feels like we're missing a part of the story. And the second is a little more subtle. We're left with tiny, odd shapes for white spece, and it's right between being either to zoomed out, or too zoomed in. It's also slightly off-centered, unlike the original, which is placed at almost exactly 1/3 of the way on the canvas.
Our eyes naturally perceive these things, as we demand order. It keeps us sane. To say it comes natural is terrible advice, but in fact the only real advice there is to give. Get used to thinking about these crop options consciously, and when you go to take your shot, don't just look at your model's eyes. Look behind them, look at your background, look at the shapes of light that are formed, instead of the details and features of a face. It's what separates a photograph from a PHOTOGRAPH.