As a kid, i agree with you. However, once you hit your teenage years, that's the defining point where you choose how you treat others and yourself. Personality is something that fluctuates. For some years, you could have a shitty personality. As you grow and experience other things, you mold your personality depending on how you want people to perceive you.
Basically, I think someone's personality does come from external factors/environment/etc, but you definitely have control over it. I know people who consider me a very happy and charismatic person. Some of my co-workers think I'm way too serious and kinda of a buzzkill.