Most Helpful Girl
I think it could be many things but some assumptions off the top of my head are that someone (or even both of the people) have experienced something in their lives that prevents them from wanting to be close to anyone and actually committing to anything. They may also be unhappy in the relationship but cannot find better and don't want to be alone so they go back to what they can rely on, which is that person who always takes them back and makes them feel good for awhile. Another possibility is they just don't know what they want so they are just going with the flow even if its not what is best for them. They could also truly have deep love for each other even though its not best for either of them to be together but they second guess their decisions to move on. Lastly, which I kind of said already, is its possible that someone is just really unhappy with themselves and so they don't think they can find anything else so they go back to what they can rely on.