Because I try. I really, really try.

One thing I see far, far too often is "dead lines". That is, lines that are one consistent weight without any depth to them. It makes the drawing look flat, static and, well, dead.

The solution is to vary line weight according to what goes in front, and what goes behind. Lines indicate change in plane. Generally, the more dramatic the change in plane, the heavier the line. Some changes are so subtle, they don't really need a line at all. But you can still indicate them with feathered lines.

You can emphasize certain parts of the form of something using this method, and it doesn't require any use of hatching or tone to create depth.

To convey something is on top of something else or in front of something else, the line delineating the edge of the form in front should be heavier than any lines of the form behind where the two forms meet. Think "rolling hills"; the edge of the hill in front should be thicker than the edge of the hill behind, and so on and so on.