It's hideously complicated and in huge part due to social conditioning. I agree a lot with HerHissyness.
Take me for example. Happy, completely functional childhood with parents modelling a very healthy relationship. Still found myself with an abuser, even though I always swore I'd never let a man abuse me. The trouble was, I didn't recognise a lot of abuse and my XP chose to wait until I was pregnant and living with him before he really showed his true colours. It started off with little things like not pulling his weight around the house. I was 3-months pregnant and thought I couldn't leave my baby fatherless over some washing up. As women we are conditioned to accept this as normal, and while it's changing slowly, there's an awful long way to go.
Trouble is, after things like that, the odd joke at your expense creeps in and you don't want to seem like you have a sense of humour failure so you laugh with everyone else, and if you tackle it you hear "but I was only joking. I didn't mean to." Then it escalates to isolation. My XP never 'told' me I wasn't 'allowed' to go out. The first few times it seemed that a genuine reason had come up why I couldn't leave him - he was ill, he needed my help with something etc. Then I would be treated to 'but why am I not enough for you? If you really loved me you'd want to go out with me instead of them". If I stood my ground and went out I'd be questioned on what I'd done when I got back. If I accused him of being insecure and jealous I'd get "But you're so beautiful - it's not that I don't trust you it's other people. I'm sorry, I do trust you honestly". Another tactic would be that he'd be stroppy and uncommunicative the day after I'd been out the night before, but perfectly fine the day after that. Before long - and the important point here is that you don't even realise you're doing it - you start making reasons for yourself why you can't go out, because subconsciously you recognise that it's just too hard work. Yet your abuser can honestly say he never once said "I'm not letting you go out." Because you haven't been hit and you haven't been called names or screamed at, you think you're over-reacting and you minimise it. Over time, this becomes so normal that you hardly recognise the escalation.
It's insidious. No man ever starts off being abusive. He doesn't wear a sign saying "Beware, I may be really nice to you to start with but once I've got you where I want you I'm going to start beating you up." Violence is at the very end of the spectrum and there are an awful lot of women in abusive relationships where violence has never featured because it doesn't need to - the ultimate aim is control and master manipulators don't need to lift a finger; they can achieve it all with a glance and a phrase. In my case, the day my XP graduated to violence and tried to strangle me, I suddenly saw it as clear as I've written it here and I left there and then. I'm honest enough to admit that I could still be there though if he hadn't got violence. Since I left, 4 years ago, I've done an awful lot of reading and talking about this subject and become very good indeed at recognising the red flags. I wish someone had taught me about them when I was younger. Sometimes, being brought up in a happy, sheltered household leaves you naive and vulnerable to abuse just as much as someone brought up in a household where abuse is normal.
Hope that helps to explain a little bit.