The law of impermanence dictates that things can’t stay the same for too long. The Buddha understood that this idea would be difficult for people to accept and that only an appreciation for the subtly of mindfulness could release us from our attachment to things. That is, only if we allow the process of change to take place while watching it can we ever overcome suffering.
The concept of mindfulness that penetrates reality as it observes the arising and ceasing of phenomena is so subtle in fact that the Buddha decided initially that he would not teach it. But a prospective student pleaded with him that there are some of us that want to see this subtly and want to learn how to let go of attachment, so the Buddha accepted.
When we have good days we want them to keep coming and we never want them to end. The Buddha warned us that this cannot be so. We cannot have good days all the time. The law of impermanence means that if we have ups we must also learn to accept the downs. This reality is inescapable which means that as long as we are holding on to what we want we will never overcome suffering. If we want to be happy we have to learn how to let go and simply observe how things change.
If we can find a way to rise above our desire to always have things go the way we want than we can be happy all the time. It’s not that kind of mundane happiness we get when we satisfy our cravings. Its the kind of happiness we get from understanding that as long as we keep bringing the mind into the present moment, we are progressing towards total freedom.