The Buddha saw existence by insights obtained through satipatthana (samatha-vipassana) meditation. He saw that the existence domain consists of three realms (lokas) kama loka, rúpa loka and arupa loka. He saw that in all the realms there is birth, continuity of births and the mind.
The mind has defilements (asavas) as its characteristics. The asavas prompt action (kamma) and produce ordeal of a being in births (jati) and continuity of births in the cycle of the existence domain.
The cycle of births leads to unsatisfactoriness (dukkha) in the mind because it is an aggregation of ultimate realities based on causes and conditions which are constantly changing and transient and which lead to arising and passing away. The continuity of consecutive births results in repeating births and deaths. By overcoming action (kamma) and results of kamma (vipaka-resultants) liberation from the continuity of existential (samsaric) cycle is attained. The births and continuity of births are sensed by the mind and the mind is subject to unsatisfactoriness.
Liberation from unsatisfactoriness (dukkha) arising due to defilements (äsavas) is attained by overcoming defilements (asavas). The Buddha explained unsatisfactoriness (dukkha) in the Four Noble Truths of suffering and also liberation from dukkha in births and continuity of samsaric cycle by following the Noble Eight Fold Path. This leads to freedom from dukkha and attaining nibbanic state of mind. When all kammas are overcome the continuity of samsaric cycle ceases and births cease to continue and when death takes place the mind attains parinibbana.
The three characteristics imperamanance (anicca) unsatisfactoriness (dukkha) and non-self (anatta) are observable in birth, continuity of births and mind. Therefore, they are universal characteristics of aggregates and also the source of unsatisfactoriness (dukkha) in existence.