At least according to the wiki, your very first response sets your personality, and then further choices of that type will "stack". So if your first response was aggressive, and then you distributed your responses fairly evenly after that point, you may have stacked enough aggressive responses to set that personality. Once it's set, you can change it by selecting another response style more often, but apparently it takes many more times of choosing that style to force a change. There seems to be a period of time at the beginning of each act where you can change the personality more easily, though.