In the case of fictional characters, the author decided their motivations but how does it work with real people? I have known people who wanted to:
have a lot of sex;
watch a lot of football;
be successful at work;
get drunk a lot;
No one chose these motivations. The motivations determined choices but where did the motivations come from and why do they differ? Although I think that meditation is beneficial, I do not buy the rebirth idea. Every psychophysical organism is a unique combination of genes. Every person is a unique organism-environment interaction and the environment is both natural and social.
With persons as with particles, nothing comes from nothing. The actual comes from the potential. Most potential people are not born because most sperm and eggs do not meet. We, the born, are the chosen few - chosen by Fortuna. We cannot change our personalities by acts of will but can begin to understand and to understand is to change.