The big idea
It's all in the title.
Why should the government be entitled to interfere when an activity affects no-one but the person performing that activity?
Why does this matter?
Any government that supports prohibition supports an outdated puritanical law that clearly makes no difference to vast swathes of people. Anybody who supports prohibition (the government included) is responsible for:
1) Inflated prices for cannabis, a 'risk tax' if you will. Straight into criminal gang's pockets.
2) No age limits for the purchase of cannabis. Black market drug dealers do not ask for ID, only cash.
3) Contamination of cannabis. Anyone who has skimmed the topic knows that cannabis is sometimes contaminated with glass/lead and various other substances to boost the weight of the product. Under a legalised system random tests could be made, and people would have an easier choice to buy from someone who doesn't contaminate their weed.
4) Mistrust in the law. When someone is growing up and they get overblown (sometimes straight false) warnings from things like 'Talk to Frank' etc, when they discover that weed isn't in fact as harmful as it is made out to be by the media and the government, why would they trust them when told to avoid other drugs?
5) The gateway effect. What little evidence there is for the gateway effect is not related to 'wanting a better high', otherwise everyone who drinks alcohol or coffee would be smack addicts by now. By forcing users to buy from drug dealers, they are exposed to more addictive drugs that are pushed because they are more profitable.
6) Criminalisation of millions. Millions of Britons are classed as criminals because they took part in an activity that affects nobody but themselves. These black marks stay with you forever.
Anyone who looks into the matter can see that keeping cannabis illegal for growth and recreational use is unfeasible, expensive, immoral, hypocritical, just plain wrong. Please Mr Cameron, Mr Clegg, don't keep the charade any longer. Even good old Dave got caught with it in school, so it clearly didn't do him that much harm!