My hair constantly changing naturally every couple of years...?

Well, my hair constantly changes. When I was a baby my hair was really curly. It was filled with ringlets. then on the course of becoming 4 it started to become straight and then by the time i was about 7 it was pin straight. Then by the time i was like 9 it started to become poofy and frizzy. I'm a teenager now and my hair is still poofy and frizzy. Its weird how it changes constantly and if there is anyway i could change it back to how it was curly or when it was pin straight. Like i know i sound like an idiot but i want to know why it changes and if there is a way to change it back. If anyone can tell me thank you....