Meeting people is (not) easy
I'd say it is a matter of opportunity more than anything else. Consider three different kinds of interaction: meeting people, making friends and falling in love. If we consider the ordinary John Doe, and not one of those rare, uber-outgoing guys and gals, what one lacks to be successful in these areas once one reaches adulthood is the opportunity. Before, there is school, with its very on idiosyncrasies and rules and codes. It is so incredibly easy to meet people while in school; there are the classes, always made of different people, and eventually you'll have to sit next to someone else; there are the common grounds; there are the transportations; there is a whole context shared by God knows how many people. And, more important, there is a natural predisposition towards socialization. We are young, innocent for a while (until we are 13, at least), and are driven by pleasure only: all we want is to have fun.
But once you leave school, the game changes entirely: you might still want to have fun (I do), but unless you're really lucky, one moment of fun costs a silly number of moments that are not fun at all. The whole environment changes drastically as well: the working world is, by definition, more defensive and more competitive. People are more wary of bonding. Besides, and unlike in school, you're seldom supposed to create your own background when you start working; you're supposed to have such a background already, with all the friends and people you might possibly need. I'm not saying that one cannot make friends, or even find a soulmate in a working context; I'm just saying that it's incredibly harder. There is the work itself, there are the routines it imposes, there is the age catching up - you know, the older we get, the less we feel like messing around. There's little time left for mingling, and even when there is, we don't really lower our defenses - we stay defensive, evasive sometimes; we don't share too much, we don't say too much, because we don't know the other well, his or her intentions, his or her behaviour, and we can take no chances. We are incredibly afraid of what others might think of us, even if we don't admit it - one step out of that line we can't see most of the time, and we'll be out of the game.