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.