The reasons why a love partner would want to retain another against their will are cultural, supernatural and instinctual.
Instinctive behaviors are strong in humans. They are part of genetic programming, which is diverse, and humans vary in many genetic traits. Desires for climbing up the hierarchy, competition, taking advantage of others, domination and control are very genetically instinctual. The culture can modify and inhibit these instincts completely, but we clearly share them with animals and these instincts are very natural for us. Check out the book "Why Zebras Don't Get Ulcers" by Sapolsky for good scientific overview on the topic of genetic instincts in love. And I repeat, although we are programmed to be selfish and competitive genetically, we are also programmed to be culturally programmable and this is my main point of the book, much of our improvement can be achieved on both the cultural and educational level.
An idea not only for loving but also for respecting the loved ones already exists in our culture. The idea to allow your loved one to leave by choice is not completely foreign to humans. I say, more cultured wiser people are much more likely to respect their loved ones enough to let them decide for themselves what they want and allow them to leave if they decide to.
As an illustration, I recently learned about an interesting observation: sitting arrangements in the car vary depending on societal level of people.
HOW TWO PAIRS OF HUMANS WOULD PICK SEATS IN A CAR? (A pair is two mates: a male and female in love with each other. In our cars, there are two pairs of seats: front and rear. Front is obviously higher in hierarchy. Even dogs sense that. When dogs are left in the car alone, they compete for the driver's seat. Now back to humans.)
THE OBSERVATION IS: IN LOWER SOCIETAL LEVEL, MALES WOULD TYPICALLY OCCUPY FRONT SEATS AND FEMALES WOULD BE DIRECTED TO REAR SEATS. (That is, in pairs MF (male and female) and mf, the seating will be Mm in front and Ff in rear seats)
(I would comment that a leader would typically be a male and would drive. Why would males take higher hierarchical level seats and leave females lower level seats? Because, in lower societal level, there is male dominance, the females are considered to be of lower hierarchical level. Part of this is because of physical dominance, part of this is because the males are designed to be fighters, and military are dominating the society and traditionally win the competition for leadership. This is very animalistic and belongs to the past of humanity.)
IN THE CAR, IF THE COUPLES ARE FROM THE MIDDLE LEVEL OF THE SOCIETY, EACH WOULD SEAT WITH THEIR MATE. (The seating will be MF and mf. The leader would be driving, and the leader could be a female)
(This makes sense because females are considered equal to males, so the factor of dominance is not relevant. But there is also a mistrust factor: partners are protective of their loved ones. They mistrust their choices, and they mistrust people of opposite sex. A female would think, "If my lover seats close to another female, they may be talking and even touching each other, so they may fall in love with each other and I would be left alone". Similarly a male would have similar suspicions. The mistrust comes from past experiences and culture. People usually experience many losses of lovers and often these are losses due to competition, so holding onto one's lover and preventing them from flirting is a very practical need, quite justified. Of course more trust and more loyalty across the population would exclude the competition. On another hand we are still evolving very much due to competition between diverse individuals for better genes. This Darwinian competition is very real or at least is generally perceived to be very real in our current evolutionary development.
The main principle of choosing a mate is:
People have limited time to find mates for procreation. Typically, it is about 10-15 years.
People have a limited number of choices - on average, it is not that easy to find anyone who would consider you as a candidate for mating.
You should look for the best possible mate matching your preferences, the preferences are usually defined by culture and conditions - often physical and financial health are most important.
Make sure that your mate believes that you are their best find.
Make sure there is enough ground to project that in the future you will keep believing that your choices were best and remain best.
Of course all these limitations are coming from the idea of lack. We are under belief that we don't have enough, that we are barely surviving, that for living, having family and procreation we need resources, food, housing and other provisions, and that if we make poor choices, we will run out of finances and will be in much trouble. These beliefs are constantly reconfirmed by our own practical experiences and by what we see in the lives of other people. In conditions of abundance of material resources, all these conditions and limitations make no sense.
IN HIGHER LEVEL OF THE SOCIETY, TYPICALLY, EACH WOULD CHOOSE A SEAT NEXT THE OPPOSITE SEX MEMBER OF ANOTHER PAIR. (The seating will be Mf and mF)
Since I am not exposed to customs in the higher level of the society, this answer puzzled me first. Then I figured out that 3 factors are in play in the higher society:
There is more loyalty and more trust to one's mate.
There is much less freedom for changing mates, there are a lot of restrictions both internal and external which prevent mates changing mates.
There may be less flirting.
There is more material abundance, so losing a mate doesn't result in much physical loss; it is much easier for the rich people to find mates than for middle class.
People may realize that they come to this world for the experience. So seating and talking to a wider circle is the best way to get more experience.