What exactly is behind this old saying? Some interpret it as a test of fate, as a check whether two people were really destined to be together. If you let someone go, that person will come back if it was meant to be, however ...
For those who don't believe in destiny, this saying doesn't make sense. But it can be interpreted in different ways. You can't force anyone to love you. You must give a person the freedom to decide and choose.
There's a small number of people who get attached to another person in a healthy manner. Most people are anxious and upset, at least to some extent. This is kind of attachment that solely comes out of insecurity. Such persons are afraid of abandonment, rejection and criticism.
Anxious attachment encourages a person to behave intrusively and longs for the company of the partner all the time. People who are anxiety attached often feel lonely and insecure when they have to spend time alone. They are dependent on the company of dear people and their partner.
When such a person is in a relationship, they are desperately trying to control the behavior of another person in different ways. For example, such a person is trying to make the other person feel guilty for not spending enough time with her. They start showing jealousy very quickly, which is another way to make the other side spend time with them.
Anxious people love being in a relationship. However, even complete fidelity is not enough for them to be able to feel secure. They are appalled by the possibility that the other side could go away.
They want to know what their partner is doing at all times. The only way to achieve this is by forcing and initiating communication. Some anxious types even use verbal or physical aggression in order to force the other side somehow to stay with them.
Although anxious people usually have longer lasting relationships with others and are almost always faithful, these relationships rarely last a lifetime, because it is a matter of time before their partner gets tired of their controlling behavior. This saying actually means that love can't be taken, owned or forced. The beauty of love is precisely in its freedom.
The more you are trying to keep someone, it is more likely that you won't be able to keep that person. Then you are not focused on their happiness, but on yourself. You become selfish, and love is anything but selfishness.
If you don't restrict the freedom of the person you love in any way and she chooses to be with you or to leave you, it's only her choice.
The only way to be sure that this person wants to be part of your life is to give her possibility to choose.
So, this generally accepted rule is not a test of love and predestined relationship, but the exam of emotional maturity of the person who gives its partner freedom.