Why is Dexter so hung up on Rita?

He wants a girlfriend in order to maintain his "disguise" as a normal person. So that the people in his life aren't constantly trying to set him up on dates, or asking him why he doesn't want to find a girl.

Rita is ideal because her "damaged goods" background means she doesn't ask too many questions, and she doesn't want to have sex (as mentioned.) If he goes to any great lengths to preserve that relationship, it's only because he knows he's unlikely to find anything so "good" (for his purposes) with another woman.

In the books, there's also another reason why he sticks with Rita.

MAJOR SPOILER ALERT. The first book is fairly similar to the first season of the show - not 100% but maybe 85% - 90%. However, the second and subsequent seasons are nothing at all like the second and subsequent books (and that's a good thing in some ways.)

In the books, Rita's children are much more traumatized than they are portrayed in the TV show. Cody isn't just a quiet kid - he literally never talks to anyone but his sister. In the third book (or thereabouts,) Dexter begins to realize that Cody has a Dark Passenger (ie: killer instinct) of his own. His sister is aware of this, because they do everything together. And so Dexter begins to toy with the idea of teaching them the Code, the way Harry taught him. This, of course, gives him a reason to keep the relationship with Rita intact which has nothing at all to do with Rita herself.

I believe the reason why Dexter seems to "care" about Rita is because he thinks of himself as an overall good person, a kind of necessary evil.

Although he lacks the emotions to be guided by in his life, he tries really hard to seem like a good person and empathise with the people around him. Dexter isn't a stupid guy, it is a well known fact that psychopaths, although unable to empathise with another person emotionally, are able to view situations from other people's perspectives and are often able to guess the emotional responses of "normal humans" under said situations.

Therefore even though he may seem to empathise with people, he really is just guessing their emotional response while feeling nothing himself.

Secondly, we know that psychopaths do not feel emotions, but that isn't always true. While there are some outright crazy and suicidal psychopaths, most of them are selfish and also have a strong sense of self preservation.

This means they may show an emotional response if they feel threatened. We have seen Dexter's expressions every time he perceives danger. We have seen him panic and make almost emotional decisions under such situations. Hannibal Lecter also shows a similar response (although not as obvious as Dexter) to a negative situation.

All in all, I do not believe Dexter's almost emotional responses when it comes to Rita have anything to do with him just wanting to appear a family guy. I believe that he seems to care about Rita not because he really does, but because he thinks of himself to be a good person and feels the need to act as one, guided in life not by an emotional sense of right or wrong, but rather the will to consciously act like and to be accepted as a good person. Here's why:

1. Dexter does not understand Rita's emotions naturally, but can often be seen trying to guess what she expects of him and then acting accordingly.

2. When faced with no choice but to kill an innocent fellow cop in order to save himself, he had no idea what to do. As we all know, the number 1 rule of his code is to never get caught. According to this, the choice should have been easy. But it wasn't, because more than anything, Dexter tries to be a good person.

Rita never knew dexter. If she had seen him over his table she would have been horrified. She would have taken the kids and vanished from his life for good. Rita and the kids were merely a cover for dexter..their purpose was to make him appear human.