We do not value friendship in large part because we do not understand what a "friend" is.
Consider Facebook, which is now the barometer of our relational lives. A "friend" is constituted by someone who you know. And while everyone gets that there is something deeper about actual friendships, the inability to identify that something deeper has emptied "friend" of any meaningful content.
It hasn't always been this way. There is an apocraphyal story of C.S. Lewis floating around that illustrates the point. A professor whom he had known for several years turned to him one day and asked him if he thought it was finally appropriate for them to use their first names in addressing each other. Lewis rebuffed him, remarking that their relationship was "strictly academic."
It's an extreme case, yes, and quite possibly untrue. But it highlights the distinctions between types of relationships that previous generations made. Coworkers were not friends and friends were not family. While this sort of clarity may have been problematic in its own right, it at least allowed each party to be clear on where they stood in the relational universe.
Now, however, that has changed. The formality, the careful distinctions, the idea of social roles has all gone away. And in their place, confusion reigns.
This is particularly true of relationships between men and women. Men have always been friendly--or better, courteous--to women, but they have not necessarily been friends. The difference between male and female and the potential for sexual interaction (a potential which only the very young or the very old may ignore without danger) was too great to permit close interaction outside strictly romantic (or in more base form, strictly sexual) contexts.
The question, then, is whether friendship--whatever that is--is the sort of relationship that men and women can engage in responsibly.
My provisional answer, which is driven largely by my experience, is that any young people seeking to find a spouse would do better (oddly) to cultivate friendships with their same sex while viewing the opposite sex through a strictly romantic lense. Keeping the roles and relationships separate allows us to have more clarity on our own feelings and behaviors in each relationship. I have seen many a person (guy and girl!) unwittingly become emotionally tied to someone who was "just a friend," only to be heartbroken when they pursued someone else.
Men and women seeking to marry should not deny the role that sexuality plays in their interaction with the opposite sex. To do so is ultimately to fall prey to a gnosticism--that is, a denial of the body--which ironically leads to a weakened ability to control the impulses of the body. Is there any wonder why affairs often start between people who claim to be "just friends?"