I was having a conversation with a friend the other day. He DP is very controlling and she is thinking of leaving, and I left my controlling XP last year. But just generally, we were musing about why we ended up with these people and how we didn't spot it from the start. Even with her relationship, I was around since the beginning and at first I only saw good in the relationship, not bad. I've seen it cited before as well, on mumsnet and other places, that controlling type men tend to pick on vulnerable women, hook them in with kindness when they are low, and then turn to control once you get strong enough to have an opinion of your own on anything.
It's almost like, man sees a kitten in a tree. Poor kitten. It's so stuck and vulnerable. I must rescue it, and it will love me forever and I'll have a cute kitten to play with. I will feed it and love it and buy it a fluffy bed and lots of toys.
So he rescues the kitten, and the kitten is grateful and affectionate, and he takes it home and feeds it and nurtures it and looks after it, and the kitten is happy, and the man is happy.
But then the kitten grows up into a cat, and wants to go out and hunt mice and prowl and find other cats to be with. But the man doesn't like this, because he liked the cat being a kitten, and innocent, and dependent on him. He worries that the cat may stray to far and get lost again, or that it may realise it can be self sufficient and never come back. So he keeps the cat indoors to keep it safe. The man is happy. But now the cat is not happy, it was a wild animal and it needs to be able to hunt and prowl at night. It scratches the furniture and meows at the door. Now the man is confused. He has given the kitten everything it could want. The kitten would most likely be dead, or a scruffy wild thing, if it wasn't for him. He wanted the kitten to stay a kitten forever. But meanwhile, neither the cat or the man are happy. The cat wants its freedom, and the man wants his kitten back.
Metaphor inspired by the Ani Difranco song "Not a Pretty Girl". It's hard to quote without quoting the entire song, but I liked this final line:
^Don't you think every kitten figures out how to get down
Whether or not you ever show up^
Sorry for the cheesy metaphor/story 
I just find it an easy shorthand. It's hard to spot the kitten-rescuers, because I think they genuinely have good intentions at the beginning. But I just wanted to say - because it's pretty much impossible to tell someone if they are already in a relationship like this (as they are generally happy, at the beginning), if you have recently come out of a relationship, or are otherwise vulnerable at any time of your life - watch out for the kitten rescuers!