"Psychological disorders" Is it Just in the mind or due to chemical imbalance as doctors say?

There are over a million people suffering psychological disorders around the globe which include many different effects., all having their own names and terminologies. Why are many people affected with these disorders? Is it just due to their uncontrollable feelings like fear, anxiety, anger, despair etc. taking the complete control over the mind or is it due to the chemical imbalance in the brain as psychiatrists say? Can people contract this disorder, just by fearing that they would get one or is it just not in our hands? And I have heard about psychologically ill people hallucinating over many things.! i.e. some imaginary person comes in front of them and speaks with them. Like, they can see dogs speaking in English and so on and so forth. Why is this so? and can someone explain me clearly about Multiple personality disorder?