There are many dangers connected with smoking marijuana, although it may seem harmless and pleasant to new users.
Marijuana may not be physically addicting, but it can be psychologically addicting. People enjoy it, so they keep smoking it. Thus they become psychologically addicted. There is hardly any difference between doing something because you have to do it and doing something because you want to do it. Either way, you're hooked.
Marijuana smokers build up a tolerance for the drug, which leads them to smoking more and more of it to get the same effect. They also look for stronger kinds of marijuana, and they can be tempted to experiment with stronger drugs like heroin and cocaine and many others which can destroy their minds and bodies.
Possession of marijuana automatically puts a person in the criminal class and can result in trouble with the law. If marijuana smoking leads to the use of heavier drugs, it can lead to really serious trouble with the law.
All recreational drugs eventually affect the brain, making it harder to think, to remember, to plan, to function, to work, to hold a job.
Using any kind of drugs almost automatically puts the user in a subclass of drug users and drug pushers. These people are often destructive influences who can cause serious troubles.
Marijuana use is anti-productive. It is demoralizing. It makes it more and more difficult to do the things that young people need to do to have decent lives, such as furthering their educations, getting into satisfying and rewarding careers, getting married, and raising their children. The pleasure of smoking pot is a poor compensation for all the damage it generally causes to people's lives if they keep it up.