In observing the contents of consciousness, we become aware that no thought is fixed. While we may have repetitive thoughts or sustained belief patterns, thoughts themselves appear as if from nowhere and then depart. This transient nature of thinking is fascinating when we start paying attention to it because when we do, we can see that our lived experiences follow the same model. In other words, there is no physical circumstance or emotional state that is sustained; just as a thought arises and passes, so do our moment-to-moment experiences.
The phrase "this too shall pass" has ancient roots. It's simple and obvious in that we all know situations and emotions fluctuate in any given moment. When our minds become fixed on a scenario, particularly a negative one, remembering this phrase helps pull us back into the present moment, the same way that focusing on the breath helps re-center the mind during meditation. As a mantra, "this too shall pass" has undoubtedly helped many people cope with difficult situations.
For me, though, I've gained the most value by recalling the phrase in moments of joy and happiness. Perhaps it is a masochistic part of my personality to remind myself that happiness is fleeing; perhaps this is the "foreboding joy" that Dr. Brene Brown describes during moments of vulnerability. The acute awareness, in the moment, that my lived experiences of joy are temporary and will be lost allow me to experience them more fully. It's far easier to accept that negative experiences will eventually pass, because of course I want them to; but surrendering the positive takes true spiritual strength. Rather than holding on to them tightly, which is instinctual, the lesson is in relaxing and letting go.
This act of surrender is what we practice when we meditate. In order to observe our thoughts, we have to let them go. Holding on to them will not allow us to experience ourselves as conscious beings. This same awareness, when applied to our outer experiences, allows us to be fully present in our lives. It gives us the courage to embrace our painful experiences so that we may process and integrate them, as well as to value our positive ones so that we don't take them for granted. By letting go, we find that there is in fact something that remains constant and unchanging - consciousness itself.