They exhibit what G.K. Chesterton called a "weird and horrible humility." To put it bluntly, we've been trained to perpetually self-accuse.
We grow up to question ourselves endlessly, to stay worried that we might screw something up. The law teaches us that we're always on the edge of being punished. All the years we spend in school teach us to fear mistakes. And unfortunately, many religions teach us that we're always on the verge of falling into sin and damnation.
The truth, however, is that we're not that bad. We just think so.
Of course, we do sometimes screw things up… but not remotely as often as we mistrust ourselves. And a large percentage of those screwups occur precisely because we don't trust ourselves!