Why is it such a big deal

She has really strong feelings on this for a reason. Maybe something happened in her family, like a relative with a drug problem or lung cancer, who knows. Or she just doesn't want to see anything bad happen to you, doesn't want you to get involved in worse things, or doesn't want you to keep hurting yourself.
(just so you know the kind of perspective you're getting, I smoke, but have a friend like your girlfriend)

She doesn't want you doing harmful things to your body, she doesn't like you when you're high, she thinks you could endanger yourself or others, and you're killing your brain cells.

Hmmm first of all to all you little " GOODIES " out there, pot helps more than it hurts, go to school and learn about it as for you girlfriend , tell her what I told my mom, " would you rather me have a joint in my mouth, or a crack pipe?"

She wants you to quit because she actually cares about you and your health and smoking pot could only ruin your life in one way or another and you should stop and listen to her because if she didn't care she wouldn't have said anything