They have them stuck in their head so much, they have to create the songs and infect others with it.

No, they can't stop coming up with them.

In the shower washing their asshole?

“Fresh all over, don’t forget / The spots you sometimes might neglect! / Soap and water make you whole, / Stay clean and proud from head to... soul!”

Jingle writers are professional ear worm avoiders. They know that to end an earworm, you need to listen to an even more catchy tune.

My brother used to do this in the 90s. No, but he was always writing and playing something.

No because they aren't annoying to them, to them its just what they precieve as an inner monologue