Why do people drink robitussin to get high?

I know it contains DXM, but is it out of desperation to feel high? My friend of 8 years has been drinking it since he was 12 and the stuff has completely ruined his life. He went through rehab and relapsed a week later. Is DXM really THAT addictive, and if so why is it so readily available over the counter? I'm not looking down on anyone who may do this. I just want some opinions. My friends behaviour baffles me, and if cough syrup is that addictive that scares me.