Why does toner always run out on the left side of the printed page first?

"Just to hazard a guess, it's because you're using English or other left-to-right languages. Since we read left-to-right, we (typically) start lines of text on the left, so the left side is used more heavily.

Some printing styles will mollify this a bit -- printing right-justified text, printing mostly graphics, printing right-to-left languages, or printing newspaper-style evenly-distributed columns.

This is why we used to extend the life of our toner cartridges by pulling them out and shaking them, often several times, before completely replacing them."