That means humans are the youkai's parents?

Well, you can put it that way

Why does youkais attack humans then?

Even though they're their parents

That is... in fact simple

That's because humans fear youkais

What causes youkais to exist

are the fears in the hearts of humans

Hearing a strange noise as you're walking in the dark and feeling like you'll be attacked...

These fears will give birth to a youkai that will attack humans

Humans creates those who eats them by their own hand