Why are relationships so essential to ones happiness?

Until very recently, each human's survival and reproduction probability was was greatly increased by having a strong social network. Since those people who were best at maintaining relationships were more likely to produce offspring, over thousands of years humans as a whole evolved to become dependent upon each other psychologically and we are rewarded with pleasure inducing hormones every time we experience improvements to our relationships. The reason a new romantic relationship is so pleasurable is that your body is trying to form a strong attachment between you and your mate which will in the long term increase the survivability and success of your offspring (particularly in pre-modern societies). You come from a very long line of individuals who wanted and were good enough at maintaining social and romantic connections to ensure their offspring were able to grow up and have kids of their own. If this weren't the case, your family tree would have been cut off long before now and you wouldn't exist.