Children have poor impulse control. They often think the world revolves around them. In general they have difficulty managing their feelings and often end up in meltdown or, worse, in violence. These emotional aspects of childishness are much of what we have in mind when we say that adults are acting childishly. And our main goal is not to insult people but to remind them that in the process of growing up they (should) have learned skills that they should be using now.
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We often deride others by referring to them as childish. Why is this an insult? What's so bad about being a child? The only major disadvantages of being a child I can think of are physiological and intellectual, and yet when we say someone is acting childish, we usually don't mean they can't perform complex reasoning or that they haven't reached the peak of their physical prowess. So how are we to understand accusations of childishness as insults?