If you believe your examiners will be fair with their questions then you’ll be more likely to try to engage with them, rather than suspect a trap or harsh comment.
If you believe you can get prepared during the submission period then you’ll be more likely to feel ready when the time comes.
If you believe the mostly positive stories about viva experiences that you hear and read then you’re going to act as though your viva will be a mostly positive occasion too.
What have you heard about the viva? What do you believe about it? And how does that help you (or not)?