I'm an introvert but still prefer to go through the human checkout lines. It's due to a combination of the anxiety of having the self checkout machine screw up and needing to flag a human over to help, plus interacting with cashiers being one of the few "safe" interactions I've come to expect. Since it's their job I can expect a certain level of professionalism and cordiality and I'm not expected to make small talk. I just smile, say hello, pay, and that's my social quota for the month.

I'm also the type of person who sits quietly in the chair while getting my hair cut and hopes the barber doesn't ask too many questions.