Here's why:
* Irresponsible implies a lack of accountability, a disregard for consequences, and potentially harmful behavior.
* Nonchalant suggests a casual indifference, which can be seen as negative in some situations, but it's not inherently bad.
* Aloof implies emotional distance or detachment, which can be perceived as negative if it's seen as unfriendly or dismissive.
* Carefree generally has a positive connotation, implying a lack of worry and a lighthearted attitude.
While the other words might have negative connotations in certain contexts, irresponsible carries the most consistently negative meaning.