The interactions created by generating life, and seeing your children grow, will change you, transform you, transcend you
so is war...exercise...having a job...anything really that can add meaning to life.
That being said I don't believe true happiness and fulfillment are possible for any human being that is not psychopathic in this world, riddled with so much suffering.
now we can agree on at least some part. i don't believe in happiness
exactly why I said too reducionist. They are not simply "default biological function". These things can come in to your life for a very large number of reasons and maybe they are a part of a drama you must live, maybe as catharsis for the frustration the quest for meaning in life entails, or for escaping existential dread.