Yeah, Stormysune's hit it on the head, there are things people like, but they don't want others to know they like.

I hide my favourites mainly because, even though it would be random people on the internet that would see them, I still wanna keep my privacy with that type of art/fetishes I enjoy, since I still think that my fetishes would be embarassing to reveal.

Entirely valid answers.

Just additional speculation here, but for artists some drawings may not quite fit their public image but it's something they like, so they fear that showing such favorites would give people the wrong impression about what their account is all about.

Aaaand then there's this gem I found.