In Germany, where I live, Marijuana is currently illegal, well not the consumption, but the possession of it. And most judges say if you consumed it possessed it at some point. While most people in Germany seem to be against the legalisation, I don't understand why.
The legalisation could help people with chronic pain, multiple sclerosis, Tourette syndrome, depressive disorders and ADHD, just to name a few diseases. Moreover the government would be able to control the sale and assure that it is not sold to children. This would also bring in a lot of money considering the possible taxes on Marijuana. With this money we could build more schools, promote the prevention of Marijuana consum and even pay back parts of the sovereign debt. Another point is that the legalisation would relieve the police and the jails (logically also save money), because the police in Germany is currently not able to fight Marijuana and every third person in jail is there because of Marijuana.
I think even more important is the duty the government has, this may be a very personal point and not everybody has to share my opinion on this, but to me the duty of the state is to assure everybodys rights and protect the personal freedom. And considering this I do not understand how Marijuana can be illegal. From my point of view this is just restricting my freedom, of course one could argue that the government is just protecting us from the "bad weed, that harms us". While it makes completely no sense to be able to buy alcohol literary everwhere. 
To actually die from the amout of THC in ones blood, one would have to smoke 600kg (~1322lbs) Marijuana in one hour.
From my point of view I do not know one argument for the ban of Marijuana, if somebody has another opinion on this, feel free to argue in the comments :)
Sources:
https://digitalcommons.iwu.edu/cgi/viewcontent.cgi?article=1036&context=uauje
https://hanfverband.de/themen/wahlen-abstimmungen-in-deutschland (GERMAN)
https://en.wikipedia.org/wiki/Medical_cannabis
https://hanfverband.de/themen/legalisieren (GERMAN)
http://discoverarchive.vanderbilt.edu/bitstream/handle/1803/5634/Limits_of_Supremacy.pdf
http://www.rdi.uwa.edu.au/__data/assets/pdf_file/0005/99905/05_14_Clements.pdf
https://www.tni.org/en/issues/regulation/item/627-altered-state