It's part psychological and part brain chemistry. Many studies have been done showing a correlation in how some emotional disorders (i.e., disthymia, unipolar, and bipolar depression) run in families. Excessive serotonin (or a lack thereof) is often implicated in many mood disorders. Similarly, brain chemistry is implicated in impulsive disorders. Studies (although not entirely conclusive) show a link between aggressive personailities that act impulsively and zinc deficiencies. People like this are also believed to also have hyperactivity in the prefrontal cortex of the brain. Emotion is the most primitive brain function. It's been shown that the amygdala plays the biggest role in influencing emotion. So it's not just mind over matter, it's a combination of both, but likely more brain chemistry than psychology. There was a study called the Minnesota Twins Study. I won't get into the methodology, but you can look it up on Google. The researchers there concluded that personality is about 70% genetic.