I've written before about the position in which I hold my camera when I'm out shooting on the street: the camera held vertically at mid-chest level, with my hands around the body and lens, and my finger on the shutter button. This automatically tends to lead to more vertical (portrait orientation) than horizontal (landscape) shots. As with anything in photography (or the arts ...... or, come to think of it, life ;-)) there's pluses and minuses for each, and there's always a tradeoff.
The first of these shots below was taken in a passageway at Grand Central Terminal. I was walking with my camera in my usual street shooting position, saw this great subject walking towards me, and fired off three frames as we passed. I had to do a little straightening of the image because the camera was on a slight angle when I shot, but otherwise the composition worked wonderfully well. The important element in this scene is that I captured the woman's entirety - head to toe. That's important here because the bent leg gives a feeling of forward motion and puts her in the forefront of all the other graphic elements in the image - the other people, the door behind her, and the kiosk to her right. It was also important for me to leave space on the left side of the image (the woman's right) to give her space to move into. Nothing disturbs me quite so much in an image such as this as to have this kind of motion implied with the person looking as if they were walking off the edge of the image and out of the frame. Using a wide angle lens to get a capture like this is critical because it creates a perceived depth of perspective. A longer focal length lens would flatten the perspective so that the door and people that appear to be way back in the distance would be flattened and look much more on the same plane as the woman and the kiosk.
In this next image, the horizontal orientation is important to create the scene unfolding around the gentleman in the foreground. I had to turn the camera to shoot this, and that simple motion attracted the attention of both men in the image. As long as I was found out, I held the camera out at arms length to get the lens as close to them as possible and fired the shutter. Obviously I did not capture either man's feet. That's not important here, but what is important is the basic rule of thumb that if you have to cut off part of a person's anatomy, do not do it at a joint - like a knee or ankle. That just makes the photo look like part of the subject's body was amputated. What was most important in this scene was to include a substantial amount of the street scene and dynamics as they unfolded - the traffic and the other pedestrians are much more part of the story here than in the previous image. The most important element is the facial expression of each of the two men, and the intensity of the eyes in each expression.