I think we've all fell into the 'well he's not like my ex so he cant be another abuser surely' trap once in a while. They come in many different packages (But ultimately, no matter how sparkly the wrapper, there's still just a big shit in a box lol).
If you read up on narcissists it might give you an idea of why they do it. Basically, they arent like you or me. Think of the playground bully who never grew up. THATS who they are. They abuse in order to feel good about themselves. It's their fuel. Though I doubt us normal folk could ever truly understand that.
They don't have any true sense of self, they just take bits and pieces from others (can manifest in mannerisms, hobbies, opinions ect). Theres lots of reasons why they may target you. Underlying all of them however I think, is that they want to take your...light I guess.
That's a bit of a hippy way of putting it...but basically they are greedy. If they see happy, confident ppl - they want that for themselves. And think they can take it by crushing you.
If they see sad, vulnerable people then they will also gravitate towards them too, as they know these people will be easier to hurt and hurting people makes them feel good.
Basically they want to take from you. They are leaches.
You may always attract them. Through no fault if your own. Some of us just do. But you can learn to spot them early and get shot so that makes life easier.
Also wise to do some reading up on codependency incase it is relevant to you. It isnt always. Narcissists arent fussy who they come for. But we have to be fussy about who we let stay in our lives.