Operant conditioning. If I pull a lever and someone in Antarctica gets struck by lightning, nothing happens to my brain. If I pull a lever and I get struck by lightning, I instantly receive a strong desire not to let that happen again. The fact that nobody but me is conditioned by my experiences is what makes me me. If I suddenly began having the experiences of another person as well as my own, that person and I would both become me; if I accidentally wandered into a giant helium balloon and died, nobody would be me. If I for some reason developed anterograde amnesia, me would have a very short lifespan; there would be no reason to care about my long-future self any more than any other person.