My sister (who is not affiliated to any faiths) asked me this question. Please do not shoot us for being egocentric, as a new student I hope to learn - and to be able to answer her.
I was telling her that all of us are just like radio wave (something I read on Ven S. Dhammika's Good Question Good Answer), and when we die, our radio wave retains our characteristics such as personality, temper, preference, etc. The radio wave then finds a fertilised egg to rebirth. This new person however, is not necessarily the same as the previous person because he/she is influenced by their upbringing and environment, forming a new set of characteristics. The argument that we need to cultivate ourselves in this live, is so that in the next life we can be born as a better person. For instance, Terasi gets angry easily this life, Terasi needs to train herself so in the next life the new person, say, Teraso, becomes a milder temper person.
She pointed out that: between Terasi and Teraso there is no memory link, nor there is any knowledge of each other, so those two are basically two different people.
And the question is: in the selfish point of view of a mere human full of defilement, why should Terasi struggles to cultivate herself just for the benefit of "someone else" (Teraso)?
I use my own name just for illustration, because despite this kind of questions, I still want to learn and practice.
Oh and, I need to provide an answer to my skeptic sister