In the midst of imperial expansion you have French Indo-china the the East, French and English occupation of China to the north, and British occupation of Burma and Malaysia to teh East and south…What’s with Thailand. As far as I can tell it was never occupied or invaided. Why?
Why Thailand has not been invaded lately is like asking why the earth hasn’t been engulfed in nuclear war. There are specific reasons for them both, but I don’t want to write the definitive work on the subject, which is what would be required to answer the question.
Maybe you want to. If so, start doing the research at:
http://www.sala.net/Thailand/history.html
I’m going fishing instead.
Eh, I’d have to pick a nit with the OP in a couple places, Occam. I realize you’re talking about the Good Old Days of imperialist 19th century expansion, but I’d have to quibble as to whether China counted as being “occupied” by the French and English. As I understand it, the Western trading nations only had the barest toehold on the country, at the ports of Shanghai and Canton.
Kind of the same thing with Burma–the Brits were only there during WWII for strategic reasons, I thought, and as soon as the war was over, they left (but maybe I’m missing something, I’m not really up on my minor Oriental nations’ history.)
Malaysia, the West was only there for a really short time, maybe a hundred years, and it’s arguable that it was never really “occupied”, just some capitalists lived there for a while. Again, I’m hazy on the details.
As to why no Western imperialists bothered with Thailand, a WAG would be that it’s got no strategic importance, kind of stuck in the middle like it is, and it had nothing exciting to trade like opium. Just lumber–pfui. But I’m the first to admit that all I know about Thailand I learned from The King and I and various Rolling Stone AIDS articles.
By around 1800, Thailand was the dominant, expansionary SE Asia state. But by the middle of the 18th century, they had lost Cambodia and Laos to the French and 4 Malay states to the Brits. The Thais benefitted from their society’s own internal stability, progressive monarchs who adopted modern policies (and appointed Welsh governesses) and the colonial rivalry of the French and British and managed to survive intact within their traditional borders.
Picking another nit, Thailand wasn’t occupied by Europeans, but it was occupied by the Japanese during WWII. I know it’s maybe late, but as pointed out so were some of the other occupations mentioned. Otherwise, the above answers are good. 
For a good historical synopsis of this period: http://www.aseanfocus.com/gateway/thailand/eclipseP3.asp