Why do human beings place such a great importance on companionship of the opposite sex?

You may be a loner. There are lots of people like you that prefer to live their lives alone. That doesn't mean there is anything wrong with dating or eventually marrying. The great importance is that humans were meant to be in family groups and be together. Same for marriage......that was to procreate and continue the human race. Stop trying to convince others. They will only consider you either gay or anti-social. You won't convince them of anything so dont bother trying,