My guess is nature, but I dunno.
At the end of the day, Homo Sapiens are also animals, even if we have developed a conscience and higher understanding of the things around us. And the one thing animals want is to continue their lineage as well as manage to survive. We're the same, even if we do have recreational sex (dolphins do this too - the only other animals to do so - which highlights just how intelligent they are).
Honestly, the main wrench in my assumption on this particular subject is the creation of birth control and the "child-free" movement that a lot of soylennials seem to partake in. But it's usually leftists that can't think of the future and can only see short-term that actively partake in the child-free movement. Their arguments are usually "I don't want to bring a child into such a horrible world" when the world is in a much better place than any other time in history, thanks to modern medicine and rapidly-evolving technology.
Anyway, that was a ramble but hopefully I got my point across all right. I've been awake since 9PM and it's now 10AM, so maybe I'm speaking nonsense.