t'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.
I'm not going to say that all smile of Thais are fake but you cannot translate them to "happy" instantly. Use your common senses to indicate how the person feels, do not include a smile as an important factor. Some people smile brightest in their sadness. Some people smile differently when they feel something else.
Even Thais sometimes couldn't understand that too and I still cannot find the answer for you why I still keep smiling all the time. Sometimes I feel like it keeps me looking nice, it encourages happiness to people around, and also it is just a simple act to show that I am happy. :)