What about the man who drowns in an attempt to rescue a stranger? Or the soldier who jumps on a grenade to save strangers in a foreign country? What self-interest are they acting on?

A desire to commit that unselfish act (self-sacrifice for a total stranger) wouldn't come out of selfish desire. Some people believe that their own life, consciousness, and self are inherently less important than those of other people, simply because one's self is experienced only by... one's self, who can choose to use it as they see fit, to extend their responsibility out to other people.