I believe it's both nature and experience. We are born with natural tendencies and are shaped throughout every aspect of our lives. Sometimes nature overcomes nurture, sometimes it does not.
I think that the factors which determine the human personality are incredibly complex, and are not limited just to us imitating others' personalities, or our major life events, but vary from small things like the media we happen to consume, the conditions we live in, to even the illnesses we contacted before and the substances we've been exposed to - whether it's the food, the air we breathe, etc.
It's a long chain of cause and effect with so many different variables that it is practically impossible to accurately predict, even more so than the weather!
And just like good health, I think that our personalities are not a given. Be thankful for being who you are today because it could always be much worse. Even the YOU of tomorrow is not guaranteed. Even seemingly basics traits like being able to feel empathy towards others, or having mental mechanisms in place that makes you feel remorse and stops you from engaging in overwhelming aggression are not to be taken as granted; not all are so lucky.
I believe that the mind is a physical entity and therefore we should do our best to protect it from harm just like we do with our bodies. History taught us that ideas themselves can be terrifyingly contagious just like a persistent virus. Puncturing the right spot in your brain can deliver a fatal personality alteration. We are all one step away from complete change, even from evil, no matter how good and immune we think we are.
One could argue that choice itself is an illusion, but maybe it doesn't even matter. I'm still going to say what I think I'm choosing to say, even if it has been predetermined!
Take care of your well being, keep yourself in a positive environment, maintain your clarity of mind and try to be down to earth. Accept reality. Try and stir your ever changing personality in the direction that you want it to be. Existence is a mysterious and overwhelming thing, and life isn't always fair, it can be in fact very scary and painful, but I think that has long as we strive to do our best, there is nothing for us to regret and lose ourselves over.
Even if it is not eternal, let us celebrate our inner world and the wonder of our personality! Like small shining stars in the night's sky...