There is no harder emotional decision than a break-up. That is why most people are procrastinating it as long as they can, so they often leave the possibility of a potential renewal of the relationship. They do not give a reasonable cause that has led to the break-up, so their partner – or even themselves – might think that there is still a relationship to mend.
Every break-up has a reason. The partners are dissatisfied, bored, or they have a conflict or all of them can happen at the same time. But if they are unable to articulate the problems, then both of them are uncertain about their future.
So, when they feel lonely, or they miss their ex, they think it is completely normal to get back together in spite of the disappointment and the doubts that the break-up has generated. They might say that they have a fresh start, but what they have is a new, toxic base. They already know each other, and they might have forgiven the hard and lonely days, but the threat of their return is lingering around them. So, the couple will be less willing to commit and to feel sated, since that toxic base is giving them constant stress.
Even though they make up, they have broken up, and that is stressful for both of them – except if one of them uses it as a weapon – so if they are unable to communicate their feelings and problems and if they cannot come to an agreement, or even compromise before their reunion, then they should accept that they do not belong together instead of poisoning each other's lives.