It could be different for other people, though. However, gambling while bearing a negative feeling is not good. Negative feeling equates to bad luck.
This is just a superstitious believe by some gamblers because it's not totally true that when you have a negative feelings it will affect your luck, these luck in question happens randomly irrespective of how someone is feeling but being anxious is just like a natural feeling that some people have when they go gambling because they are scared off losing.
There are some times where we are feeling like something bad is gonna happen but then nothing really happens. Anxiety is not always logical. Sometimes you just feel anxious for no reason at all. You just do. It doesn't always mean that something bad is gonna happen. Unless history shows otherwise. I know people really who have a very good gut feeling and they can always somehow feel when something bad is about to happen.