This is a deep issue. Let me try to point out some key facts.
1. Everything in this world (living and inert) have origins in the mind. “Dhamma” means “to bear” and they bear everything in this world.
– Dhamma are energies produced by the mind. Basically, they are created by kamma vinnana.
– This is why “Manōpubbangamā Dhammā..”
2. Another way to say that is to say everything arises via the Paticca Samuppada (PS) process.
– We have discussed how future lives arise via the akusala-mula PS.
– The arising of non-living things is a deeper aspect of PS.
3. Whether living or not, all are sankata.
– That basically means something that arises and destroyed and is subjected to unexpected change during its existence.
4. Living beings (there are an infinite number of them) create future lives as well the external world that is needed for their “enjoyment.”
– However, due to those intrinsic problems associated with a sankata in #3, neither a life form nor those “external things” cannot be maintained to their satisfaction.
5. Therefore, suffering is intrinsically “built-in” in this very complex process.
– Striving to gain “sensory pleasures” in such a world is fruitless and dangerous.
– That is what is embedded in Tilakkhana: anicca, dukkha, anatta.
6. We can get a simple idea of why a living being is a sankata.
– We have discussed how kamma vinnana leads to the arising of a “mental body” or a gandhabba.
– That gandhabba is created totally by kammic energy. It is unimaginably small energy.
– But once merging with a single cell (zygote) in a womb, it can grow to be a full adult. See the first several posts in “Origin of Life.”
– When that “coarse human body” dies, that gandhabba can make a few more. But by the time the gandhabba itself dies (when the kammic energy for the human bhava is exhausted), the mind has created kammic energies for many more such “mental bodies” for various existences.
– So, at the end of a particular mental body, the mind grasps a new existence. That is how the rebirth process continues.
– Of course, most of those existences are in the four lowest realms, because of the tendency to do immoral things to “gain pleasures”.
7. Only when one can grasp the inherent problem with the above process, that one will attain magga phala, and stop grasping new existences (bhava.)