I think that it's an extension of their own guilt and shame. It's a "misery loves company" type of thing. In an attempt to rationalize their own behavior, they try and mirror their faults on to their attachment(s).
Here's a good example: a while back while happy in my r/s with my ex, we were laying in bed on a Saturday morning. We'd probably just had sex, I dont' recall. But out of what seemed to be the blue, she says "I think that we should live apart so that we can date properly". I thought "What the heck?". Here she is being negative just like that. Laying here, happy and what is this crap? Probably led to a big fight. But here it is now a couple of years later and she was absolutely right. We never did date. We just jumped right into a r/s. She knows what she "wants". She knows what is right but for reasons far too complex to explain, we both jumped into a r/s without going through the dating ritual. The timing was crappy but what she was saying was from her subconscious consciousness and in hindsight; she was right. As a Staff Sgt in the Marines used to say to me "good initiative, poor judgement".
