I believe the mindset is only obvious is tourist area. Majority of their businesses will be tourists who also behave similarly, i.e trying to bargain down the price as much as possible.
Most of the tourists comes to thailand once or twice in their lifetime so they will not see you again, thus it might be his only chance to make more money of you by ripping you off and never see you again.
Once they are able to see you are a returning tourists, aka goes there 1-2 a year at least, then its a different story. A totally different story if they know you are living in thailand i supposed, but i have no personal experience on this. Anyway, due to this particular reason, if the guy is someone i really like, which is very few, i will give a second try and almost always the second one is much better.