Books and Parents

So, every few months or so I finish reading the books that I bought, and i ask my mom or dad to take me to the book store to get more. Most of the time the types of books that I want to read are ones like; Go Ask Alice(about a conflicted teen who trys drugs and runs away a few times), Crank (about a teen who starts new addictions to crank and starts an alternate personality), and Tweak(about a boy who grew up addicted to meth). Whenever I find the books i want my parents will read the back cover of the book or ask me what it is about, every time they tell me that the book is "pretty morbid and heavy stuff for me to be reading." And they try to talk me out of getting it, but end up loving it and wanting to talk to them about it and they tell me that i need to read something "happy". Personally, i find happy books to boring. I mean wheres the excitement! the things to relate to? Why do parents care what type of stories we read? If a book was too upsetting for us to handle, wouldn't we decide that, and decide if its worth not reading?