Here’s my take on it. In high school, college, and the early working years it is easy to make friends because you see the same folks, doing the same thing every day right along with you – taking classes, going out for extracurriculars, working up the corporate ladder. If you’re lucky, you mature/start a family and it’s no longer so cool to hang out at the bar watching a game with your buddies while drinking a ten-pack.
The older you get, the less you’re around new people to make new friends with, and you’re a little more picky. I have a harder time making friends these days but I find taking classes and continuing to do stuff like play tennis keeps me around like-minded people.
One of the best feelings in the world is to be OK with being alone (unless you’re the Unibomber, that is)!