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.
Maybe it's a misconception of people, especially because of the excitement of gambling results. It may not be a correct idea. Is it possible to win even if you keep your mind happy with the wrong bet? I basically believe that winning a gambling bet does not depend on being anxious but depends entirely on luck. If you have good luck, you will win the bet and if you have bad luck, you will lose the bet.