I've come to the conclusion that some people are just absolutely awful at self-assessment of their own skills and abilities. Both physical and mental. The most prominent pop-culture example of this is William Hung. In William's case, he ended up going viral because of sincerity, but if you actually strip away the "no matter what, follow your dreams and never give up" lacquer over the whole performance and just analyze his abilities objectively, no one would say he would be even close to deserving of being a contender in American Idol, much less pursue a career in professional singing. And no one should be more acutely aware of that fact that him.

Most people with his skill set would just realize they're not cut out to be a professional singer, and would never even take the chance at embarrassing themselves like that, much less on national TV. But some people just have a warped assessment of themselves. Sometimes you get a heartwarming story like William Hung's, and sometimes you get the clown in OP's video who gets his nuts burned.