Following the line of reasoning in Correspondence Bias, because it's probably much more likely that someone who seems to you to "be an angry person" has just had a bad day.
According to our current understanding, significant mood altering mutations are much less common than many other more probable causes of anger. This is one of the reasons gene therapy is not typically suggested as part of treating anger management issues.
Wouldn't it be interesting if everyone had exactly equal hormonal tendencies toward various emotions?
"This particular episode of angry behavior is not as strong of evidence that this person has angry tendencies as my brain wants to treat it" is not the same as "Angry tendencies do not exist at all."
If it's worth saying, but not worth its own post, even in Discussion, it goes here.