Like many others here have written, our definition of purpose seems to result from an extremely complex and potentially infinitely extensible set of factors including:
many levels of genetic factors influencing capabilities and pre-dispositions;
many levels of environmental factors (many of which have random aspects);
many levels of cultural factors, including the stories that we have heard that had emotional impact for some reason;
many levels of chance;
many levels of analysis and understanding;
many levels of choice, commitment and habit.
The evidence from modern science is clear – that we are all complex embodied entities that are complex beyond our ability to comprehend in detail.
Any understanding we have of ourselves is (beyond any shadow of reasonable doubt) a gross over simplification of the realities that we are.
We are all, always, part mystery, even to ourselves.
And we are capable of choice and action and making a difference – at least to some degree.
The more conscious we become of the many ways in which we are predisposed to certain classes of action, and certain classes of error in detection and classification of the world around us and ourselves, the greater the degrees of influence we can assert in reality.
And reality must always be far more complex than our limited comprehension of it. So there is constant need of both effort and humility in our choices and actions towards whatever purpose we have (to whatever degree we have chosen it and/or accepted it).
And choosing a purpose that is in our own best interests, and the interests of our family and friends, and those of our neighbours, and our communities, our districts and humanity and the ecosystems within which we exist, and any potential wider communities of sapient entities we may encounter, is important.
Being responsible in as many contexts as we can manage seems to be what is required of us if we are to survive.
And we are dealing with complex systems, and simple rules are never appropriate responses to complexity. Complexity demands an iterative approach, as each time we interact with it the response is likely to be different in some set of subtle and not so subtle ways. So we need to observe and adapt, ongoingly. That is the only sort of response to complexity that is viable long term.
And that can be difficult, with our limited abilities to comprehend, our limited models, our tendencies to simplify in order to make some sort of decision, and our tendency to rely on habits and patterns from the past.