Why do people have different personalities?

Why do people have different personalities?
In 1833 there once was a girl name Nia she was 15 years old she was a very nice girl but at times she had harsh speech that may hurt others people didn’t understand why she was that way. People said she had split personalities and for a moment they thought it was no one like her on this earth until one day she meet this boy name trey he was a charming, easy going romantic person. Trey did have a dark side he was easily influenced and flirtatious. A scientist from London thought they was the most two amazing people so he decide to study there character he did research for days. He came up with this idea saying that Nia was born under the sun saying that she was a gemini and anyone was was born