I think what we’re seeing with Quinn’s fascination with Carrie is the effect of observing someone else. Carrie has this deep, intimate bond with Brody and it started before their romance. It started because she watched him in his home, at his job. She spied on him and observed him and it created a feeling of attachment and intimacy. She feels very connected to Brody to the point of not respecting his boundaries. Carrie always seemed to me to be more attached to Brody than Brody did to Carrie.
Quinn has been tasked with watching Carrie in the same way more than once and its reasonable to assume that the same bond or connection is developing as a result. He’s used to knowing everything, hearing everything and seeing everything so its no surprise that he is attached to her and has no problem snooping.
I think its brilliant to have Quinn attach himself to Carrie like Carrie attached herself to Brody, but with Quinn its a bit less intense because he is just a less emotionally intense person. They’ve done a good job exploring that this season and they really ramped it up this episode by giving Quinn access to such a private piece of information and shown how it impacts his relationship with Carrie who, naturally, doesn’t feel the same bond to Quinn than he does to her.