A very interesting topic!
Why do some folks 'doubt' and others don't?
Doubt= lack of belief that an expectation can be met.
Do some folks expect better performance than they're prepared to train for?
Do some players think they can control the outcome of a match?
At each level of play, a player can only expect to perform as well as they have prepared. This does not determine wins and losses only the performance of the individual. Removing the expectations and treating the game as a journey (working to learn and improve) rather than a destination (winning) can all but eliminate pressure, improve performance and enhance enjoyment of the game for all involved.
..-)Almost sounds like a Nadal presser, eh?