Lots of replies here about after life, and just doing the right thing because you're supposed to or "empathy", yet there are a set of people like OP who only observe this when their life is put in front of them for review, maybe they do need religion?
This is a reasonably assertion as far as it goes.
At the risk of being a dripping faucet, I'm poking at "Why?", given an inevitable return to the dust from which all came.
Possibly I'm guilty of over-reading the word.
Often the question "what is the purpose of my existence?" is a proxy for some less abstract question, I think. Consider Young Frankenstein, and the gag where characters sing "Oh, sweet mystery of life at last I've found you! At last, I know the secret of it all!" because they got sex. Less cynically, it may simply be a matter of identifying comfortable values, in terms of the possible values available in the human condition in the present day. I mean you're unlikely to be honestly asking a question with a giant universal scope, if you claim that it bothers you personally.
This Destiny is in tension with Free Will (in my telling).
In retirement, my hope is to produce a lengthy, pretentious exploration of a few ideas that will doubtless help someone's insomnia.
Free Will, in this telling, exists on the logos/eros plane of the individual.
Destiny is some target on the plane out there at infinity toward which one navigates over time.
The origin of this coordinate system is the heart, and one may or may not hold a connection to the Creator via the thymos.
Otherwise, it's just so many chemical reactions.
Most people judge themselves against a narrative that matters to them. Most people do not want to cast themselves as a villain in their story.
You may ask "but what does it matter if we are all dirt". It matters to them, even if there is no godlike perspective above it all. To be honest, I'm not actually sure why having an afterlife or some super-being would create any more explicit meaning for an individual life.