"Be more careful" is meta-advice, most people don't actually start trying to be careful until they recognize the need. Worse, and I don't understand why, but they often need to be reminded of it again in different situations, that is the need to be careful or to pay close attention seems to be context dependent.
I'm mostly asking this open question to those among us who are well-versed in developmental psychology (I'm mostly thinking of children) . Although, failing the actual scientific research on the topic, I guess some testable hypotheses would be great too.