Everyone talks about Minnesota Nice, but what is it? In a nutshell, it's being nice when you really would rather not. It's being passive-aggressive or attempting to guilt someone, all while passing desserts around.
This comic is a great example; granny offers you a bar (which is Minnesotan for brownie or pan cookie) because no one was eating them at her church function. They filled up on Harriet's bun sandwiches!
You have to say yes or granny's feelings will be hurt.