For people with religious convictions, I understand: your god(s) hate you if you have relationships not sanctioned by your religious authorities. I am specifically asking about secular people in the US and Europe, who choose to enter a contract with the government about their personal lives.

Is it taxes? It seems much easier to me to just make more money than to make "till death do us part" work.

For health insurance, domestic partnerships work closely enough. (This is a US-specific problem.)

It seems to me that most marriages occur because the wedding industry has done a phenomenal job of marketing itself. Many girls grow up dreaming about their weddings. Men, assuming that women want this, go along. With predictable results in the divorce rate.

Companionship and love have nothing to do with a piece of paper filed with the local government.