Emotion is an important factor in many types of photography, and controlling the direction of the dominant lines in your photos will help you when you want to create a mood. This effect will be strengthened if other components of the image such as lighting reinforce the effect of lines, e.g. soft, warm lighting with restful lines or dramatic lighting with dynamic lines. There are three line directions to consider (clicking on a photo will enlarge it):
Vertical lines give a sense of strength, and here the solidity of the objects reinforces this.
Diagonal lines are dynamic, lively and energetic, giving a sense of motion. Here the man's pose contributes to the dynamic feel because it is obvious that he is working (work is output of energy). Also, when objects in an image have the potential to fall - in this case the arms, upper body, and diagonal pole - they add life to the image. I cropped the photo so that the pole went from corner to corner, which harmonised the diagonal with the frame and gave the strongest possible diagonal line.
Horizontal lines create a feeling of passivity and restfulness. Here the warm, soft, cosy light works together with the horizontal lines.