A fetish removes the "human" from someone and turns them (or whatever the fetish is) into something purely sexual. It's a hypersexualization.
If someone says "I like nice feet on a woman" that is a preference
If someone obsesses over a woman's feet to the point where they are infatuated with "the feet" it's a fetish.
Basically, a fetish dehumanizes someone and only obsessively focuses on a specific trait for your sexual preference, with no regard of the humanity of the person attached to the trait
