Some people have on - off relationships. That is their dynamic. They are all in love for a time, then its all off.
Why does this happen? - Generalisations.
Passion: The people are passionate and fiery. They can exaggerate things, and feel very high or very low.
Regret: They feel extremely sorry for their words or actions and want to make things right.
Memories: They conjure up the good memories, dwell on them until they are bigger than the initial problem - the thing that split them up in the first place. They think that because they have a history - this will help patch things up.
Inequality: One partner loves a lot more than the other.
Loneliness: They are lonely and think 'better the devil you know'.
Convenience: The relationship has been established. It is too difficult to go through the process of finding someone and dating all over again.
Dependency: They have got themselves into a rut. One may feel they cannot do without the other.
Manipulation: One or both are controlling. One or both enjoy 'the dance' or 'game'. Power struggle.
Making up: Relive the courting process.
Drama Kings and Queens: One or both are getting something out of being in 'crisis' mode.
It is not really for anyone to judge this kind of relationship - like people in the world - relationships are made up of all kinds.