According to tradition, The Buddha taught the core of his teachings, the Dharma, during his first sermon. These teachings were not written down until four centuries after his death. The orthodoxy of Buddhist traditions was, and still is in Theraveda Buddhism, maintained through a series of coucils, the first of which took place a week after the Buddha's death. Life is suffering. This is not a pessimistic but realistic statement.
Life is imperfection, impermanence, emptiness.
Busshism does not deny happiness but acknowledges that happiness is transitory. Suffering is also associated with unpleasant persons or conditions, with separation from loved ones and pleasant conditions, with not getting what one desires. Dukha comprehends all forms of physical and mental suffering. I as an individual is only a convenient term applied to a combination of five groups that are never the same from moment to moment. There is nothing permanent behind them.