For centuries, Thais have had a hostile attitude towards the people of Myanmar (formerly Burma) due to a long history of war between them. There is a deeply rooted feeling of bitterness that's been passed down from one generation to the next.
It is not an exaggeration to say that such perceptions have been emphasized in Thai history books, drama series and movies that portray Myanmar and its people as villains and our enemy. The seed of hatred has been sowed for so long that a number of Thais tend to look down upon and maltreat labourers from Myanmar.
Before my first visit to Myanmar earlier this month, I was among those who felt "not so friendly" towards Myanmar since I had read a lot of books about the fall of Ayutthaya to Myanmar in 1569 and 1767. Some books and drama series portray invaders from Myanmar as looting and burning down our former capital, killing many Thai people and siphoning off valuables from Ayutthaya to their country in 1767. Many Thais are convinced by what they've read and feel terrible about that painful chapter in history.