I would argue that one’s responsibility for one’s own happiness ends and one’s responsibility for others begins when one cares about the others. In terms of social situations, one accepts responsibility for another’s feelings or actions when one cares about the other. For example, if your friend does something bad and gets in trouble, you will probably try to defend them. On the other hand, if someone you do not know or do not like does something bad, then you probably will not jump to their defense. This is the border between where one's responsibility for one’s happiness ends and where one’s responsibility for other’s happiness begins. In Into the Wild, this can be seen with Chris and Carine. They comforted each other when they were young
