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

People usually date because they feel lost and all alone and just need one person to hang onto. However they usually make irrational decisions when choosing a mate and that is when heartbreak happens. Alot of it has to do with hormones in teens. As for sex, it is the amount of a certain chemical in your brain being released at one time, which is why people do it.