It's not just people in a service industry, almost every Thais here are smiley. I am one of them and I've never questioned myself why I keep smiling too.
Smile is a common thing to have here. A non-smiley person is often judged as not a friendly person. To be with someone and you don't smile might seem like you don't want to be there or to be with him/her. So we may use them so many ways more than other countries.