My reason against marijuana.
Why do many people claim to use it? To be 'happy'.
Not that I know from experience or anything, but articles and other users (some on this forum) said that it does something with chemicals that go into your brain and make you happy, just as anything else that makes you happy or whatever.
So then, here's why I'm against that.
You should earn happiness by accomplishing something, solving a problem, or being given a thoughtful gift (whether it be materialistic or not). You shouldn't be able to just turn away from these things just because you can light a plant on fire and act like a senseless child. It's also very evident that everyone I know that does marijuana is mentally slower than everyone else. I'm not sure if this is a proven fact or if it's actually a side-effect, but it's something I've noticed. I'm still wondering if it's the drug that makes people handicapped, or if they only did it in the first place because they were handicapped.
People taking it for medical purposes is fine.