I'm stuck on this for now ...does the Buddha say anything about why we exist ( mankind ? I know how much he stated about reasons why we should not exist such as Nirvana being the highest goal to escape from the karma bond!! Just like to know for balance of reality.
The main point of paṭiccasamuppāda pretty much comes down to: "if this exists, that exists; if this ceases to exist, that also ceases to exist". And so the reason to our existence, not just mankind but pretty much all conditioned phenomena is that the condition or the root cause for that existence still exists. So in our particular case, the reason we, mankind, still exists is because the conditions to our existence, the 3 unwholesome roots of greed, hatred, and ignorance, still exist. And as long as one has not completely uprooted them, s/he will continue to wander in Samsara's 6 realms of existence.
'Existence' ('bhava') is a mental 'view' ('ditthi'). When the mind is free from 'self-view', 'existence' will cease. Instead, there will only be mental & physical 'elements' (dhatu) rather than 'existence'.
By & large, Kaccayana, this world is supported by (takes as its object) a polarity, that of existence & non-existence. But when one sees the origination of the world as it actually is with right discernment, 'non-existence' with reference to the world does not occur to one. When one sees the cessation of the world as it actually is with right discernment, 'existence' with reference to the world does not occur to one.
Why now do you assume 'a being'? Mara, have you grasped a view? This is a heap of sheer constructions: Here no being is found.
In terms of 'physical life', this comes from the elements & from mother & father.
...this body composed of the four elements, born of mother & father, fed on rice & porridge...