Several factors:
1) Kids aren't required / encouraged to socialize in person any more by schools. One example: Dates are no longer required for dances...and dances in general are becoming much less attended. In my day, requiring dates for dances got me a few free opportunities to meet new girls...to practice dating by being forced into real dates even if I was shy. When one of my buddies was dating a girl at an all girls high school, but he didn't want to go without some guys he knew, he would just set me and all our other buddies up on blind dates with a half-dozen girls from the school. I would do the same for him. Voila! Socialization. Today? That never happens. Packs of girls go to those dances without dates.
2) Technology: Kids socialize through technology instead of in person. Kids (boys at a minimum) play video games on Friday night instead of going to parties. Kids don't try to organize their Friday night outing during the week by talking to people at school. They send a text. Maybe they get a reply; maybe they don't. They start a text chain, and then get elminated without knowing about it, they stay home. Even in college, dinner at the dining hall is often organized by text, not by walking by an open door in the dorm. And open doors in the dorms are becoming few and far between. That's considered weird.
2a) Technology -- Porn. The need for a teen boy to have a girlfriend to release his pent up sexual frustrations is as low as it has ever been in the history of mankind. Marriages are down. Reproduction in general is grinding to a halt in every country which has ubiquitous access to the Internet.
Granted, these are first world problems. They are still problems. And, this does lead to depression and suicide, and pointing out to our kids that they should feel lucky because they don't have to run from the lion anymore doesn't actually solve the problem.