@kara_dreamer actually the optimal children's/manga/anime source here is Shigeru Mizuki (gegege no kitaro), who really understands these things, or did while he was still here. he is an example of someone who had many experiences and manages to convey them in a really clear way.

yes I can easily imagine positive reasons! and some yokai aim at the positive, some don't, and some (even those) are simply alien ...