The Buddha said life is suffering (dukkha). Suffering is the condition of unsatisfactoriness. Immediate pain and sadness is of course suffering. But in happiness, in avoidance of pain and sadness, we are still in the condition of suffering.
To understand this and to change one’s mindset one can end suffering or end the illusion of suffering. True “happiness” is not temporal happiness but a seemingly transcends time and space. By extension even this true happiness is illusionary. Nonetheless we must pursue it as if it is there even though it is not there.