Thai people literally can't say no and it's pretty evident in their language. However, it's not just a language thing that they can't say no because it's also associated with the people's attitude. It just shows that Thailand is indeed the Land of Smiles because of their accommodating and loving attitude towards other people.
According to BBC, there's no word for "no" in Thailand. Chai means "yes" and the opposite of that is mai chai which literally translates to "not yes." Not saying "no" directly could mean anything that's why Thai people literally can't say the word "no."
But what's interesting is that the language is complemented with the same kind of attitude. Thai people are always concerned about how others feel rather than thinking more about themselves. If you asked someone a favor in Thailand and they would say mai chai, it's usually accompanied by a sad gesture with a small bow and eyes filled with regret for not granting your request.
If a woman is speaking to you, it would be mai chai ka and if a man is speaking, it would be mai chai krub. Thailand Breeze also stated that adding ka/krub at the end would make the sentence more polite. These polite words added at the end really makes everything clear that Thai people really can't say no.
This kind of attitude is greatly influenced by traditional, cultural and religious beliefs. According to Reach To Teach, Thai people are mostly Theravada Buddhists so most of them are non-confrontational and they strive to maintain a positive attitude despite frustrations.
Most countries really say what they want but Thailand doesn't. This is one reason why a traveler needs to be a good listener because people might say yes, when in fact, they're saying no. For example, people might say yes to your request but they would, later on, explain that they have other plans or things to do.
It's really interesting to know that this country literally can't say no. But what's more interesting is the attitude that they have that complements the language. This explains it why Thailand is one tourist destination that travelers really want to go to.