Arfarfa I've already addressed your point about "man meets woman, man goes unexpectedly mad - trigger?" in my 21.11 post, but I'm happy to expand on it.
If you are brought up in a dysfunctional, unhappy family, that will feel normal to you. So when you meet someone who makes you vaguely unhappy, you might not like it, but you accept it as normal. Often, it's only when you start wising up and realising that your parent's relationship was dysfunctional and left you emotionally unequipped to form a functional relationship, that it dawns on you that the adult relationship you are in, is in fact, as mad as that of your parents (though not necessarily in the same way).
One brilliant example I know of is a good friend of mine who last year had counselling because of her problems with her father. During the course of her reading and counselling, it hit her like a ton of bricks that her family life had been hideously mad and that her marriage was simply wrong - she had fallen in love with her DH at a time when she was an emotionally fucked up young woman still suffering from the after effects of being brought up by dysfunctional parents. So she divorced him. (No children involved.) He was utterly bewildered, couldn't understand what he'd done wrong, didn't want things to change etc.etc. To the outside world, it looks like she split with him for no reason at all and had broken his heart out of some kind of selfish, frivolous, "grass is greener" agenda. But to her, the realisation that he is not the sort of partner who will make her happy or whom she can make happy, meant that the only course of action which was honourable, was to divorce him. He just wanted everything to go back to the way it was. But the way it was for her, was being insecure, control-freaky, unhappy, feeling worthless, having eating disorders, etc. That was no longer acceptable to her, because she had learned that she didn't need to feel like that for the rest of her life. All their joint friends have dropped her because she's "in the wrong". But she knows she's not and so do I.