Every one of us experiences joy and sadness, both large and small. Every one of us experiences the mundane nature of life, as well as its transcendent nature. In each of these cases, and many more, every one of us would do well to remember the secret of life in four words: This, too, shall pass.
It is a cliche that people tell the survivors of a tragedy, that "life goes on", which is true, although not always helpful at the time. When we are in the immediate grip of a great sadness---the death of a loved one, the breakup of a marriage, a devastating natural disaster---we are often so overwhelmed by our strong emotional reaction, that we cannot conceive that our life can ever go back to "normal" again. It feels like our grief, our anger, our terror, are all that we have left, or ever will. For most of us, though, in time---sometimes a long time---we regain our equilibrium and resume some sort of normal life again. We don't forget the bad thing, but we make some kind of accommodation
People don't often say to someone experiencing a great triumph that "life goes on", although it is still true. A friend of mine recently had a novel published, to excellent reviews and good sales. He complained to me that, despite the success of the novel, he found he still had to deal with the mundane, irritating aspects of his regular (non-writing) job. Part of him, at least, wanted to enjoy his success to the exclusion of "regular life". I had to remind him that "life goes on" in the face of great joy, too.
"This, too, shall pass" suggests that nothing we experience in this life is permanent. Our great tragedies will not last forever. Neither will our great triumphs. Even our mundane, boring daily chores will not last forever. We will have all these things at different times, and in different measures. While immersed in any of them, we allow ourselves to believe that it is a permanent, steady state, but, truly, we know that's not so. We sometimes need to be reminded of this. Good friends are those who know when we need reminding, and do so, even when we don't thank them for it immediately.