In my book "Deployed: Survival Guide for Families at War" I describe many aspects of codepenence or what Bowenian therapists call being not differentiated. I drew several models to depict the unhealthy relationship. I think everyone has need to become more differentiated. 
I draw for my clients first an oval with a stick figure inside. The oval represents their independent sphere or domain. Inside of the boundaries of the oval they are responsible for all things independent like eating, hygiene, work, friends, hobbies, sleep and more. A small baby can then be drawn in the stick figures arms to represent the absolute dependence of the baby on the parent for eating, sleep, clothes, and shelter. 
Hopefully as an independent being we meet and marry another independent being, but what happens when you unite? Do the circles overlap? If that's what you thought, then you have just become enmeshed. Who is responsible for what now? 
If the circles overlap on one another then who is responsible for how much you eat? Can you blame your spouse if you overeat or oversleep or dress poorly? Yes, you could, but it would be an unhealthy codependence. 
Both partners would be dependent upon one another emotionally. Emotions should be kept within independent domains. They can be discussed and supported, but only the independent person can actually heal or process his or her own emotions. 
Even if I accidentally pushed a person down the stairs and begged their forgiveness I could not heal them. Similarly if someone is emotionally hurt I cannot heal their hurt. 
I draw a healthy marriage as two independent ovals inside of a larger oval that expands their domains. Within the larger oval they share childcare, house chores, vacation decisions, bills and money, sex, and many other things. 
A critical task for all couples is to differentiate what tasks are individual independent tasks, and which are interdependent tasks that need discussed and negotiated. 
Let me know what questions you have about this. I usually go on and on about this model because I like it so much!

Sociopathy, as I understand it, is dangerous in counseling because the person, who lacks a social conscience, adapts to counseling and begins using the counseling lessons to become more manipulative. It has been my unfortunate experience to have counseled several sociopaths. I only realized it recently when I noticed a trend in several couples I had seen over time. In my cases it was only the men in whom I identified the behavior. In the beginning sessions the man openly criticized his wife with blatant hostility, then after weeks of discussing why this is detrimental they stopped criticizing for several weeks and became perfectly compliant.

The wife would then start to feel safe to share things and open up significantly. I would be pleased and declare the relationship improving. Then the man would start criticizing me in subtle and then more blatant ways. Then the man would adopt an attitude of being the victim. Then this is the part where I think the sociopathic behavior appeared because they would introduce to counseling the idea that their wife was abusive and that they could tolerate it no longer. The scary thing is that they would use wording to describe the abuse almost word for word as the wife or I had used to describe his behavior in previous sessions.

As a therapist this is scary because it suggests my useful perspectives could be twisted for ill purposes. I remember being counseled once while at LDSFS that if we identified sociopathy that we should stop counseling because it was contraindicated and could make things worse if counseling continued. 

Brandon Sanderson the author of the Mistborn series was described by Orson Scott Card as incredibly wise. I have learned so much about human behavior from reading Brandon's books. The Mistborn series is about a world of people who gain powers from swallowing bits of metal. One type of metal allows for people to soothe others' emotions. From the perspective of one of the main characters Brandon describes how the emotions of individuals can be raised and lowered but that all the emotions are present at any given time, they just have to be raised or lowered. 
I pictured something like the equalizer picture below. I think Brandon's observations are accurate that emotions are all present at once usually but they raise and lower and clash and aren't always synchronous. Emotions like sadness, hurt, anger, jealousy could be high while happiness, surprise, delight are low. Like a song all elements of music or emotions are present at once but in small degrees.