Because they get too curious about sex and what it will feel like, so they try it out for themselves.
I guess they're anxious, but when they think back they're going to realize they should have waited till they got older.
Because guys are horny. Girls are horny. One pressures the other, and they end up having sex. It's "cool."
I had a girlfriend when I was 17, she was 15.
Her mom got her birth control pills , we both grew up just fine & have no regrets at all.
I loved her very much, that was back in the 70's
people in the US are much too concerned with sex while in the meantime the country is going down the tubes. It's sex & nothing new here.
People forget they were young once themselves!