Perhaps the fundamental truth of being is that of solitude. We are born out of a union of two, but we, the child, are a unit of one. And when we die, no matter how many people surround us, or how many people that we love are near, we still enter into the next life/world (or we just die and cease to exist) alone. Depending on how you look at it, every action could be a reaction to the desire to exist in conjunction with another. Sex, the physical (and sometimes more) union of two bodies (and potentially more depending on your personal perspective(s)), could be nothing more than two people reaching out to each other in as primal a fashion as possible. "Love" and the idea of "love" could also be thought of in this fashion.
We are social creatures. That is what anthropology and biology and psychology tell us. It's absolutely unavoidable. But what these subjects have not questioned, to my knowledge, is why we are social creatures. Aristotle believe that the city, or polis, was the highest state of being. Multiple people working towards the same goal in the same manner. People, so he thought, were not complete or true until they entered into a political partnership. Now, regardless if Aristotle is correct or not, he never broaches the question of why we cannot be complete until we enter into a partnership, political or otherwise.
Perhaps one or some or all of us find fulfillment within another person, but even though we have that moment of respite, it still ends in the same lonely fashion. Naturally, this isn't supposed to discourage the idea of love, rather to support it. Even though we are born and we die alone, finding that kind of partnership very well might make the whole shebang worth it wouldn't it?
