[...] in many areas of life, incompetent people do not recognize—scratch that,  cannot recognize—just how incompetent they are, a phenomenon that has  come to be known as the Dunning-Kruger effect.  Logic itself almost demands this lack of self-insight: For poor  performers to recognize their ineptitude would require them to possess  the very expertise they lack. To know how skilled or unskilled you are  at using the rules of grammar, for instance, you must have a good  working knowledge of those rules, an impossibility among the  incompetent. Poor performers—and we are all poor performers at some  things—fail to see the flaws in their thinking or the answers they lack.

David Dunning, after whom the Dunning-Kruger effect is named, wrote an article diving into some of the ways ignorance increases our confidence and makes us act foolishly: We Are All Confident Idiots