The human eye scans in 'saccades': straight-line jumps between points in the image field. But if there is just a single point in the image field, then the eye will jump between that point and parts of the image frame.
As the distance between a sequence of points decreases, a line is formed. Lines are the second-most basic graphical element and the first element put to paper during illustration. Furthermore Gestalt perception's Principles of Continuity and Completion, based on the notion that the eye has a propensity to forge straight lines, imbues lines with the attributes of direction, angle, length and flow.
Photographically, lines are often implied when there is less control over the shooting environment. Lines which exist literally are predominantly the edges of objects - perceived by the contrast in tone and/or colour on either side of the edge.