Why do they do that?

I am learning about what the teenage years are going to be like. My oldest son has an excuse for everything. He doesn't want to do anything that doesn't have to do with his disc-man or his scooter or his friends. The sad part is he is only 10 years old. Like last night he pulled a fast one and didn't practice his viola, he came up with every excuse in the book when I was confronting him about it, on my lunch break. His teacher didn't give him something specific to practice, he didn't have his book, he didn't have to practice for 20 minutes only 15, and on and on. I was so frustrated I wanted to through him across the room. Ofcoarse I didn't but I sure wanted to. I don't know why they start acting this way and I don't know if I am up to the next 20 years of teenage brain damage that I have coming.