I imagine that for some it is peer pressure/bullying.
For others I think they genuinely just want to do it because they are just getting to an age where they become properly aware of sexual activity and become most fascinated by it. Probably makes them feel rebellious as it's something they know their parents/adultsin general would disaproove of.
I spent all my teenage years throwing tantrums and having arguments about how I wasnt a stroppy teenager...