Here's a few things:
Rights - You should be able to decide whether or not you want to smoke it. The government shouldn't tell you if you can or cannot.
Cost - It costs a lot to have marijuana illegal. I remember someone putting it this way once, we're spending money on the "War on Drugs". It costs a lot to keep people in jail (food, health care, etc.). Basically, we could be spending this money on more important things.
Making it illegal does NOT make less people smoke it. As cileskot said, it's a way to be rebellious. Also, it's easier for kids to acquire since it's illegal. If there were regulations like with alcohol, it would make it harder for kids to get. (there are dealers all over the place, whereas to get alcohol, they need a fake ID, etc.)