Question: Why do teenagers always shift the blame?

I’ve just come back to school after nearly three years of being locked away in my room (long story, I’ll explain later), and we’ve started a group project; I did all I could to finish my part but it wasn’t put in, for one reason or another, but I’m being blamed for it because I wrote the piece. I don’t understand.
What happened was I had to write an article and give it to the group leader, but she was away, so I gave it to her friend, who gave it to her (I think) and the group leader gave the full project to her friend, and it was given to the teacher. Unfortunately, though, two pieces were missing, but the slot where mine should have been said “Article handed in too late” with my name in asterisks. It was ‘too big’ (it was freaking a third of a page WITH a 36pt title), and I didn’t know at the time, so I couldn’t do anything else about it, and it’s apparently my fault; I’m being dissed for it, when I edited it a heap, and made it as small as possible.
I’m just wondering why teenagers – girls especially – always shift the blame onto someone else. Does anyone know? I don’t know what I could have done, and I’m kinda wishing I’d stayed locked in my room for the rest of my life, because everyone hates me…or am I just soft?

Thanks for listening to my rant guys. See ya. =)

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