Contrary to others spiritual sources, Zen Buddhist literature does not specifically answer this question but rather look at the attributes and attitudes of what the practice should be and what attachment means.
In the Sutras, the practice of love can be summarized as follow:
Wanting others to be happy in achieving permanent well being using an altruistic, non-conditional and
unselfish behavior and without expecting anything in return.
It includes support, compassion, generosity, understanding, non-judgmental and patience.
One of the best ways to explain how our mind works is t use the analogy of a movie theater.
When we are watching the giant screen in the dark, we are almost totally absorbed by the story, plot, characters, script, sound, music, momentum and special effects. Our thoughts and feelings are connected and manipulated by the 2D/3D images on the screen. If the movie is powerful, we are in it and we believe in it at least during the show.
The images and characters on the screen exist because we see them but are they real? Of course not.
“Letting go”. 2 words which carry an automatic negative meaning such as “giving up”, cowardice and even failure. To let go without resisting with a good fight is simply not acceptable in our secular Western society. Obviously there are situations where resisting and fighting are totally justified such as defending a good cause or defending your position when facing wrongdoing for whatever moral, ethical and socio-professional reasons.
A part these extremes,“ letting go” that is “accepting as it is” should be considered.
The Buddha said many times “ I am teaching only two things: suffering and end of suffering”.
If we are clinging, that is if we do not let go a situation/ feeling that is out of our control or without a solution or both, suffering in whatever form is unavoidable because we will carry our emotional struggles day after day.