When we are in long-term relationships our world view changes and we normalizing being in a relationship. We normalize behaviors, thought patterns, conversations, mental states, etc…. After a period of time we no longer question it and simply live our lives constantly receiving information that says that everything is normal because this is what has become normal. This is now the state that the brain will seek to achieve and you will begin to unconsciously put effort into maintaining it.
When a relationship ends we are faced with new information that is in stark contrast to our existing world view – that which we and our brain consider normal. This means that there are going to be a lot of unconscious automatic behaviors aimed at returning your mental and physical state to how it was when you were in the relationship. The body seeks homeostasis and it resists deviations from normal.
Given this, it makes a lot of sense to push reset at the end of a “normal changing” relationship and take the time to make “not in a relationship” the new normal. If you don’t, you are going to manufacture the same experience and likely engineer the same unsuccessful outcome for the next relationship.