I need some info regarding my friend's father who has been suffering from some behavioral disorder for the past couple of months. Actually, that was all I was told but I did hear that he has major issues with trusting people and that he even developed paranoia. What is the basic reason for mistrusting people in paranoid persons?
Problem with paranoid patients is exactly what you have described. Thinking that he is being watched, followed, that there is some conspiracy agains thim going on, those are all the symptoms of the paranoia. Of course, it is treatable, but the proper therapy has to be set and the patient should be monitored as much as possible.