Alright, here's my argument.
As many have said, legalizing marijuana will significantly help the government reel in money by taxing it. Secondly, it takes the power of dealing away from criminals and reduces organized crime. And, honestly, a lot of the bad changes of marijuana (such as modern strength of the drug) was caused by its illegality. It's similar to the 18th amendment, banning alcohol. People will still drink, you can't stop that. And through that, the proof of alcohol often increased, and organized crime as we know it began. Hopefully, it's just a matter of time until government officials and voters realize that.
The main counter-argument is that it is a "gateway drug," and that it leads to harder drugs like cocaine and heroine. Yes this may happen, but that is to the fault of the customer. You're not going to stop people from smoking marijuana. It happens everywhere, and in fact is basically legal in California, you just need a cannabis card. Whether it leads to other drugs is up to each person. So, why not take advantage of the money and the reduced crime?
If anyone has anything to add or argue, I'd love to hear it.