Mentioning previous answers is just intended to provide a fast, decent answer. It is not to discourage further questions or discussion, so if you get followon questions or want to debate a point, please do so.

"How did Thailand manage to preserve its independence throughout the Age of Imperialism while the more powerful Burmese and Sikh empires failed to do the same?" at https://www.reddit.com/r/AskHistorians/comments/5biy9a/how_did_thailand_manage_to_preserve_its/d9p1k3s/ has an answer by /u/Flocculencio . It's pretty brief but looks quite authoritative.

There's a longer answer in "How did Siam (Thailand) avoid colonization by Europeans?" at https://www.reddit.com/r/AskHistorians/comments/1dc20m/how_did_siam_thailand_avoid_colonization_by/ , by /u/sterio . (I will studiously ignore someone's pointer to Wikipedia. yodatsracist was younger then, and so were the subreddit rules.) They touch on the diplomatic / buffer state reason of the answer linked to above, but add another about state restructuring.

"How was Thailand able to avoid European colonization and the subsequent revolutions of the 50s, 60s, and 70s?" at https://www.reddit.com/r/AskHistorians/comments/3sac91/how_was_thailand_able_to_avoid_european/ , by /u/anonymousmousmous and Omegastar19, appears to echo the above themes, but with some more details in areas and a citation.

"Why was Thailand never taken over as a colony?" at https://www.reddit.com/r/AskHistorians/comments/72mta3/why_was_thailand_never_taken_over_as_a_colony/ by cee2027: I think it doesn't add anything new, but at least it's a quick read, so you might check for something I missed.