sometimes genetics, sometimes its a physical health problem that affected the persons brain and their brains functionings, sometimes drugs and alcohol can cause a persons brain chemicals to change and not work right....I know someone what was hit by lightening, perfectly normal before it but after his brain functioning and brain chemicals were not working right any more resulting in his having PTSD, generalized depression and bipolar disorder.

line in the sand there are many reasons and things that can cause a person to have mental disorders due to chemical imbalances.

suggestion your treatment providers can help you understand why your brain is the way it is, assuming your question is about yourself. if its about someone else well their treatment provider will know whats what behind why their brain works the way it does.