First of all, I have to say, brilliant question. This is one of those questions I look at with excitement, wondering what I could decipher in my mind from trying to provide an answer to it.
I've had the tab open and I'd been mulling it over in my mind now for a while.
The first thing that came to my mind was companionship. But at the same time I realized that is not entirely true. That the real answer, in my opinion, lies in first examining the exact opposite; the opposite of companionship, which is solitude and the opposite of being in a relationship- -which is being alone.
Solitude is known to have a lot of advantage. A lot of writers have made it the major theme of their numerous books, but perhaps no writer has written so searchingly into the question of solitide like Gabriel Garcia Marquez, the Latin American Nobel Laaurete, in his book Hundred years of Solitude.
Here it might seem that solitude is a confinement, but still a lot of people prefer solitude to companionship at times. When alone you get centred, you can think and be present and really enjoy some things which you can't in the presence of others. Also most of us are at our truest self when we're with ourselves. Knowing all these then, we can wonder why then do we require relationships?
Allow me to offer a few suggestions regarding the things most people seek:
1. Fulfillment
The truth is whether we like it or not, we are all a part of society. We might yearn for solitude once in a while, but the urge to socialize will always be present. And the society is set up in such a way that to find fulfillment for an individual mostly depends on his or her relations with other.
John Donne said it better, He said No man is island, entire of itself. There will always be people who know better and have seen and learn more than you do. Most of the times we make mistakes and the way our subjective senses percieve things might not actually be the way things are (of course there are people who argue there is no objective reality at all). Either way, to be certain of some things, you must look for validations from others.
Are you trying to be rich and successful? You must go through other people in a way. Are you trying amass knowledge? You must learn from others. Are you trying to learn how to find peace alone? Ironically you must also relate with people in some way or another to get this..It is mostly ineveitable.
2. Joy and Happiness
Even though there is a joy in solitude, there are perhaps even more joy in relationships. Most of us know the absolute indescribable joy of having a partner whom you love with all your heart. A person who you can share your sorrows with and tell everything to.
Most of the times we're never happy without this person and when they arrive their presence is like a rainbow and our joy knows no bound. You feel alive, more alive than you can ever be alone.
Some lucky ones even experience this with ordinary people they come across everyday, with friends and families and colleagues at work.
3. A Sense of Belonging and Purpose
Two heads are better than one. When a collection of men comes together, history has proven, there is very little they can't achieve. A man might think he has a cause or purpose on his own, but when he comes together with people of similar reasoning the cause becomes stronger.
We all yearn to be a part of something greater than ourselves. Something to which we can point and say proudly, Yes, I am a part of THAT. It's fun to say yes I did that. I did this. But it is even more fun and more fulfilling to say Yes, WE did this. Yes WE did that.
Also - sometimes we have breakthroughs on our own, but it never means anything without people to share it with. (Which is why the internet craze of sharing every moment of life is so paramount. But that is a question for another day.)
4. Moderation
Finally I would like to end with a note of caution. Yes no man is an island and no man knows it all, yes there is a great joy to be gotten from getting along with your fellow man. Yes we need validation from people, yes without people sometimes success have no meaning. But doing everything in excess is terrible.
There is no easier way to secure the greatest sadness for yourself than by always seeking validation from people for everything you do; to never be content with yourself or your success until you've shared it with the whole world.
Sometimes it is just better to sit alone and enjoy your success over a cup of wine while you ruminate to yourself.
As I said, like most endeavors in human life, moderation is the most paramount. Knowing the middle way; doing just enough without going overboard! Thanks for reading. Cheers.