Zen is telling us “When you are living realize also that you are also dying”!
The vast majority of people who listen to this statement would react almost violently: “This is terrible, too pessimistic, too morbid.”
On the contrary Zen is simply teaching us to live our lives plentifully and to be aware of our own ineluctable mortality.
If birth occurs death will follow. No way out. Fear of death and aging are our most powerful fears and resisting or denying them is one of our greatest causes of anxiety at our subconscious level.
Death may happen anytime since our life and everything else around it is transient and unpredictable whether we like it or not. This is not a morbid and macabre thought, just the reality that we have to face and accept.
To avoid subconsciously this reality is pure denial and illusion based on strong ego generated self-preservation.