How do people learn to love? Is it innate? Or does it have to be nurtured in infancy? And if it wasn't, are you basically fucked?
I'll tell you how I see personality. I believe everyone has a core that radiates joy, love, and peace. From the age of 3-5 it shines pretty bright. As you get older, language develops and needs (for status, recognition, power) become stronger and more pressing. Thinking and worrying eventually take over and covers up that core.
In most people, that core comes out to play around certain people or during certain activities. Family, best friends, (some) psychologists, and comedians can help shore up worrying and thinking and expose the inner core. Likewise, artistic projects, contemplation, and Flow can expose the core. In other people like psychopaths, the core gets buried deep but I believe it's still there. (I don't think anyone can really tell whether it is or it isn't. There's some evidence that psychotherapy can help psychopaths, but how and to what degree is hard to tell.)
A good upbringing can help expose the core by helping kids develop mental resilience, Trust*, and calmness. That makes them available to connect to others and care for them, from one core to another. A checkered upbringing can bury the core under layers of confusion, self-doubt, and worrying which then requires excavation by developing mental resilience, Trust, and calmness. There's no research either way, but I would guess that 99% of people fall somewhere in between those extreme. Of those, lots of us, myself included, are looking to make contact with that core through all sorts of techniques and tricks. That's what mysticism is really all about.
*"Trust" is capitalized because I'm talking about universal trust.