Buddhism teaches that nothing is permanent. The first noble truth of Buddhism is that there exist great pain and suffering or dissatisfaction in life. It is not a pessimistic view, but more a realistic view.
Life is constantly changing. Nothing is permanent. You may be well today and ill tomorrow. Alive today, dead tomorrow. When things doesn’t measure up to our expectations, we become depressed, angry or sad.
If we learn to accept the fact that life is impermanent and subject to change, then we will have some insight into what gives us true happiness and what doesn’t. If we are able to do this, then our suffering or dissatisfaction will diminish.
Traleg Kyabgon explains:
Normally we think our happiness is contingent upon external circumstances and situations, rather than upon our own inner attitude toward things, or toward life in general. The Buddha was saying that dissatisfaction is part of life, even if we are seeking happiness and even if we manage to find temporary happiness. The very fact that it is temporary means that sooner or later the happiness is going to pass. So the Buddha said that unless we understand this and see how pervasive dissatisfaction or duhkha is, it is impossible for us to start looking for real happiness.