And if we're not 'punishing' ourselves in this form of verbal assault, we might hear it out loud as little kids: "You should know better," or "You know better than that!" Why the latter may be worse: Were it not for others saying it to us or our otherwise learning it as a pseudo-valid form of contrition, would we really have ever adopted the belief in the first place? Rather than shaming ourselves into learning some sort of lesson - which is quite insane enough - others are trying to do it (to us STILL) as adults. And the language may vary but it still falls under shame.
In this context, "I should know better," or its twin, "I should have known better," are phrases said in the wake or aftermath of perhaps a series of unfortunate events or something that did not turn out as we'd hoped.
