The only perspective I can offer is one that relies upon the neurobiology of love/romance. Love is as much a biological drive as thirst and hunger. Humans are mammals, and mammals are vulnerable to the elements and predators, so they need the protection and resources associated with social groups. Subsequently, we are wired to experience social isolation as psychologically and emotionally painful, which motivates us to pursue relationships. As well, infants and children are dependent on their caretakers much longer than other animals; children are more likely to survive if both parents are present to provide protection and resources. So, when two people enter a romantic partnership, their neurophysiologies create a powerful state of desire for one another. Their dopamine levels rise, which results in a subjective state of "need" or "craving" (when dopamine levels rise, physical energy and cognitive focus increase, and this helps an organism to pursue whatever it needs at the time, whether it is food, water, sex, chocolate, love, etc.). As well, new relationships are marked by a decrease in serotonin levels, which is thought to be associated with the obsessive thoughts about the loved one. Low serotonin is also found in disorders such as OCD and depression, so this high dopamine and low serotonin characterize the euphoric misery of a new relationship, where you need less food and sleep, and where all of your energy seems to orient toward the love object. These processes can last as much as two years before things return to normal - long enough for a pregnancy and for a baby to live its first year of life, the time period when the child is at most risk of death.
The neurobiology of mammals, including humans, motivates partners to stay together to ensure procreation and survival of children - whether or not the relationship is healthy or whether a child is born from the union. Essentially, your mind and body come to need the other person as it needs water and food. While the emotional instability of new relationships will eventually subside, you will continue to need that person because your neurophysiology has adapted to that person’s presence in your life; they literally become a part of you, reflected in the re-wiring of neural pathways associated with self and body perception, emotions, and memories. If the relationship ends, your body responds by increasing your desire for that person, referred to as the "protest phase" of break-ups. Your dopamine rises and your serotonin falls to levels more extreme than when the relationship first began. This is why you keep thinking about the person, why shared places (a restaurant you ate at, or a store you shopped at) bring a rush of emotions and memories, and why your body aches to be reunited with that person, even if the relationship was unsatisfying. These experiences are even stronger if you were the one who was left behind or “dumped”. This is also why you simply cannot “erase” a romantic relationship from your life. You can get rid of all physical reminders, but your mind and body will still crave that person as it craves oxygen. Unlearning your need for your ex takes time because it takes time for neurophysiology to change. This is why it is so important to be discerning when choosing a romantic partner. For better or worse, that person will change you.