I have only smoked once or twice, and I had a cold so I could not get very high, but, for one reason or another, I have done a lot of research into marijuana. It is illegal for the same reasons alternate energy sources are illegal. It is a much cheaper alternative to many things (this) and big business does not want the legalization of marijuana to harm their profits. Also, some people have simply been brain washed into a bais of marijuana or straight up lied to about the effects. It has very little physical down sides (assuming in moderation) that can be avoided mostly depending on how it is used (i.e. When you light a plant on fire, any plant, it produces various amounts of tar in the smoke, but different smoking techniques limit/eliminate the tar. Also, the chemicals that get you high are bronchial dilators, opening up your lungs and letting more tar out, tobacco does the opposite). The worst experience I have heard of is one of my teacher's friend smoked often and one day got from an untrusted dealer and it turned out it was laced with cocaine. She had a heart condition and had a heart attack. She survived fine. This is part of the reason I am pro-legalization. People will smoke either way, but if marijuana was regulated by the state, then it would be safer to do.