That may be true. However, some kids can also develop the sense of being responsible either because of the nature of the people around him (this can also be developed even if there are caring adults around) or because that's is just what is taught to him like how parents/teachers teach children manners. There is also the possibility that he himself had experienced a similar gesture of caring and kindness and is just mimicking it now as he knows that it is the right thing to do. If i may also add, based on experience, i did develop some sense of responsibility when i had younger siblings of my own. I didn't have to raise myself or something like that but it was just like a switch in my head or smthng like that