Buying a vehicle is not generally a pleasant experience.
The goal of the sales guy is to find out what you like and don't like, and then show you what he has that you will fall in love with based on what he knows. He wants to know what you want as a payment - but that's only to pass on to the closer. He wants you to fall in love with the vehicle - you have to have it.
Once you are in love with a vehicle, he gives you to the closer.
They assume that everyone lies about the payment and look to push that number higher. It's all about the monthly payment to them. If you love it, you'll pay whatever it takes - that's the assumption.
If you already have your financing lined up (eliminating the monthly payment game) and are looking for a set of features but not falling in love with the vehicle on the lot, you have taken away their best elements. They get out of their game and it becomes a matter of "what can we sell it for". The sales manager and financial folks deal with that. so the sales guy and closer are mostly out of it.
Keeping these things in mind can help.