why is it that kids or teens get trouble for their actions and school?

Why do kids or teens get discouraged by parents? I mean why do they have to be so mean all the time? This is how i feel. I feel like a disgraced to my mom and sister. I feel like she compairs me to my sister all the time. Or when i have a bad grade she gets mad at me. If it is one thing that makes her happy is that she wishes that i would talk more. I feel like i can't tell her how i feel about it. Every time we fight it is just well hard for me. She is so judgemental in my point of view. I mean it is bad enough that she already gives me a hard time. She makes things worse than they are. I feel so out of place. I feel like a piece of trash.