Friendship is ... True? Precious? A Lie?

Why do people seek somebody else's company? Why do they try to be friends with somebody?
Because they enjoy the other person's company? Because they find the other person interesting or stimulating? Because the other person is somebody they trust or confide in? Or maybe because otherwise they'd be all alone?
Are these good enough reasons? The first three surely are - they are the basis every good friendship is built on. How about the last one? If you have nothing in common, have interests and views that differ so vastly that you disagree on almost everything, have a fundamentally different approach to life even and one just bores the other out of her wits - can a friendship really sustain itself on the basis that there's nobody else? Surely, very few friends really agree on everything - a certain difference might even be refreshing. But what if the only thing that holds the two together is that they need a friend? Just somebody that is willing to listen to the other's sorrows and worries and frustrations and problems. Merely somebody that is at all willing to spend time them.
It strikes me not so much as a friendship but an alliance: an alliance against loneliness maybe. And by its very nature it will only be renewed if there's a reason for it. It will ultimately collapse as soon as one of the two finds somebody that will not only fill the gap but will also add something more to the relationship. Somebody that she would want to be her friend. What about the one that's left? It's a race, a bizarre competition. The winner takes the trophy. The loser is left in the dark.