For example, Cinderella is realistic in the portrayal of the stepmother and her daughters and their ill treatment of Cinders. Resentment, dislike, hatred even of anything not connected to blood kin does happen and more often than perhaps we would care to admit.
Then there's the topic of pride and what that can lead to when unchecked. Go to Snow White's stepmother for the tips there! Pride/vanity can and has led to people trying to destroy those better than themselves (and sometimes succeeding, sadly).
As for unrequited love, The Little Mermaid could tell you all about that. Anyone who has ever been a victim of bullying because they look different would sympathise with The Ugly Duckling.
A thought occurred to me recently as a result of a discussion I was having on Facebook about the left behind disabled child in The Pied Piper of Hamelin. It was asked whether we felt the child felt left out or had had a lucky escape. Opinion was pretty much split down the middle, I fall into the child feeling left out camp, but it occurred to me that Hans Christen Andersen was ahead of his time here in recognising the disabled can and do often feel left out and was highlighting that. Sadly, still relevant. I would like to think one day it wouldn't be because we leave that "leaving out" state behind us.
In the meantime, I think we need, and will always need, the blunt honesty of fairytales showing up what we can be like.