because you know which one it is.
I believe we have 2 main dichotomies in our psyche. Instinct/survival which is more of the selfish level and morality (doing the least harm), which is our altruistic side.
When survival is difficult, selfishness usually prevails. As society becomes more stable, our selfish instinct hopefully allows our altruistic nature to flourish. Being stuck in fear, which many religions use, keeps us from our potential in the latter.
Marriage has traditionally been about survival.