What this sentence tells us is not to care too much about the gains and losses of life, but to be broad-minded and treat life with a peaceful attitude.
When you leave your original place, your relationship in that place will fade immediately. Metaphor is that the world is cold and human feelings are indifferent. "People leave tea cool" is a natural phenomenon. It is also the biggest feature of this era. How much kindness, how many beloved people, if not around, if not often reminded, will turn a blind eye, over time has become a thing of the past. I remember Tolstoy, a great Russian writer, has a vivid metaphor to the effect that friendship is like a pot of boiling water, which will gradually cool down once it leaves the stove.
What we have to do is to read the big book of life well and increase our knowledge.