Why?

Why is it that when you come prepared, and someone else isn't, you become the supplier for them? My gosh! It is school! There is a reason the teachers give you a supply list at the beginning of the year, and then give you two weeks to get it all in! If you have a mechanical pencil, and need lead, wouldn't the most logical solution to that be to get more lead? Why do you not come prepared to the one place where it matters you be prepared? She needs to stop wasting my mother's money, and start buying her own darn lead! And her own paper. And pencils. And eraser. She usually asks for either lead, a pencil, an eraser, paper, or all of the above! I want to say no when she asks me, but in the south, it is not "customary" to not share every single thing you own. I'm tired of it! It's my mother and father's money that pays for my supplies, and when she uses them, she's just freeloading! I mean, once-in-a-while is okay to ask for stuff, like when you ran out suddenly. But when you're going to ask everyday, it gets really annoying. When I say no, she has the audacity to complain that I am mean and stingy. Yea, we're the best of friends (or so she claims).
But whatever... enough with that. I have so much stress, I don't know how to relieve it.