What are the reasons why a individual may use a form of communication that is not based of there formal lauguage system?

the are so many reasons and the list is endless the persons could be intoxicated with drugs either medicinal or recreational ,they could have sensory impairment like hearing and sight .in some situation it could be that the person has to much stress and have shut themselves of to human interaction or they have mental health issues .others could be because they hearing voices in their heads .