I sometimes wonder if anyone has ever done a sociological study on the most judgmental groups of people in our society. Surely mothers would be somewhere near the top of the list. It isn't an inherent thing, of course, but there's something about the way society treats moms that encourages unhealthy competition. Sanctimony replaces understanding and, before you know it, you're writing an article about all the times you've judged other moms and why you should admit it.
In retrospect I must admit that my judgement of mothers started way before I ever became a mother myself. The first mom I judged was, of course, my own. Then I started looking around and judging my friends' moms. Eventually I started meeting moms close to my age and the judgements grew harsher. I wouldn't say anything about it to others, but I definitely had some fairly negative opinions of, and biased assumptions about, these folks who had chosen to keep the species going.
So why all the judgement? I can't honestly give any one specific answer, except that it must be something learned and reinforced by a culture that expects all mothers to be "perfect." So perhaps the best way to begin advocating for and supporting all mothers is by recognizing all the times I've judged them, so that I might better understand then instead.