I just wanna do my purchase and get out, I don't want to have a conversation so the self-checkouts are a godsend to me. Plus, I usually have to do two or more purchases as well (my stuff, fiance's stuff, this card that card whatnot.) and it (almost) physically hurts to have to say that out loud to a real person who is like, lookin' at me and then the person behind me makes exasperated noises and that makes me nervous and then I mess everything up and drop things.
I'm not good with real life people.