Lord Voldemort faced his downfall because he chose to believe in Trelawney's prophecy about him losing it all to a boy born on the last day of July. He took it seriously. And it turned out to be true.
This is how most prophecies work. Whoever is concerned hears about what is foretold and then takes it seriously and tries his hardest to not let it happen and in the process, leads up to what was foretold. This is a common theme from the Mahabharata to modern fiction.
While this is an extreme case of taking a prophecy seriously enough to act in ways that eventually makes it come true, the case we see and experience more often in our lives is of a reverse kind. Where we, time and again, fail to take ourselves seriously, resulting in not seeing through what we are hoping to and are dreaming of.
We are dreaming all the time, and having new ideas all the time, and are constantly pushing them aside, making room for something else that seems more pragmatic and something else that is entrenched in the society as conventional wisdom. We are constantly pushing down our thoughts for not wanting to offend anyone, and hence fail to standup for what we believe, either because we tell ourselves that our views are not important enough for everyone to pay attention or because we don't want to draw attention to ourselves and have to defend our point of view.
We only like being put on the spot for things that we truly care about. But the funny thing is, we only truly care about things regarding which we take ourselves seriously.
Unlike self fulfilling prophecies where we only strongly believe in someone else's prediction, the oft experienced case is one where we bring about mediocre performance in something that we convince ourselves to not really believe in and not take seriously. How many times have you held back what you wanted to say in a meeting? How attached are you to the decisions taken in such meetings? How driven are you to see them through by doing all you can?
Answering those will tell you what the impact of not taking yourself seriously is. You lose interest over time, you don't feel any conviction to see things through and you end up going through the motions and delivering something that is mediocre at best. Has that ever happened to something where you have stood your ground, been confident and tried everything you can to convince others of your viewpoint? You give it everything you've got when you take yourself seriously. And that may not always mean that you will end up achieving what you had dreamt of, but you will not regret the effort you put in, because you know that you did everything you could.
And deep work is only done by people who take themselves seriously. Because, without that, you are sure to throw up the white flag and surrender at the first sign of trouble. Taking yourself seriously is nothing but committing to your idea and having the desire to see it through.
Take that away and you are merely following someone's instructions. Take that away and you're not bringing anything new to the table. Take that away and you can't be counted on.
Because you don't believe yourself that you can be counted on.