While I'm no biologist, I do think Dawkin's Selfish Gene gives a good blueprint for what motivates "life" in general: Spreading itself no matter what. The vast majority of life is being kept in check by geographical and other constraints, but we humans mostly have managed to bypass those constraints, at the cost of massive environmental long-term debt.

So the smart thing to do, as supposedly the "smartest beings on the planet", would be to come up with another, maybe even sustainable, reason for existing than merely "propagating ourselves" like there are no limits to those finite resources we are mostly wasting to do that, at least right now.

In those rare cases where we decide to agree to "live for bigger and more complex reasons" we often manage to build amazing things; Like whole Empires, a "Land of the free" or space stations zipping across the skies at 17.000 miles per hour. Imho that's one of the reasons why people like Elon Musk can have such an, sometimes nearly cult-like, impact: He offers visions for a "unified humanity goal" without using the clutches of religious dogma or much nationalistic fervor.

His ideas appeal because they still hold true to your original "biological cause" for existing: Colonizing Mars would be an epitomal achievement for "terrestrial life".

There's no reason. If rabbits overeat their environment and they die of hunger, and humans do the same on a planetary scale, it means we're not even as smart as rabbits, don't you think?