Growing up I was the scrawny, weak kid that didn't eat much. When pubery hit my weight got normal but I was skinny-fat and still very weak. I never liked sports or any form of exercise; I didn't even know how to ride a bike. Despite all that, I was kind of agressive and secretly enjoyed getting into fights (I never started them, for the record.)
At around age 13-14 I got a friend who was very athletic and relatively strong even though he was a bit younger than me. I had a huge crush on him and wanted to be around him and be like him so I learned how to ride a bike the day after I first met him and made it my goal to not be a weakling anymore, because I wanted to be able to do what he and the rest of his friend group (which became my friend group) did. I started Googling things like "how to get stronger", "how to do a pull-up" etc, I started attending kickboxing classes, spent many hours researching nutrition and working out.
So yeah, that's why I started excerising and I'm really glad I did :)