He actually did a market study and realized that his videos with that smile on the thumbnail got more views. Like, a lot more. So he quietly started adding it into every thumbnail. He even went back to old videos are replaced the old thumbnails with ones that had that smile.

As for why that stupid smile makes his videos more popular? No clue. Kids are weird, and he does well in that demographic. So maybe it’s just a meme at this point. But the reason he’s known for that smile is ironically because it makes him more money.