It doesn't have to be appealing, it has to make you click.

Car crashes are not appealing, and yet it is something most people are tempted to look at. Many people think of dopamine as the pleasure hormone, not really, it is the motivation hormone, pleasure is one way to achieve that, but so is horror.

It makes evolutionary sense, if something horrible happens, you better pay attention, to get prepared so that it doesn't happen to you.

I don't know the details of the psychological response to Mr Beast thumbnails, and I think neither does My Beast himself, the analytics say it works and that the only thing that matters to him.