Earlier quoted context omitted.
What's wrong with "because of social and evolutionary pressure"?
"Evolitionary" implies some direction and execution scope, does it not? 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.