Questions about example sentences with, and the definition and usage of "Obsession"

A: He has an obsession with cars.

Gambling has become an obsession for him.

A: His obsession with video games is a little concerning. That is all he does.
She has a real obsession with her new boyfriend. They are always together.
I have an unhealthy obsession with eating hotdogs even though I know they are not healthy.

A: She has an odd obsession with eating grapes. John was obsessed with his new toy.

Obsessed just means that they like it very much and are spending a lot of time on it. Like when I say “Katy has an odd obsession with cats.” It means that Katy really likes cats and spends a lot of time with them

A: Obsession, it's the same word

A: × "My Past Obsession" as a music playlist name
✓ "My Past Obsessions"

That's what I would say but you could say it differently.

A: I have an obsession with mixing colors and materials.
I never get tired with it.

A: Watermelon should be plural ("Watermelons")