I know weed isn't nowhere near as strong a drug as cocaine or heroin or that other stuff but I don't use it just because I have seen what it has done to friends. I have no doubt the vast majority of people will have no ill effects, but I've had some friends that basically became stoners that smoke weed in their room 24/7. Too many to be a coincidence. I think it is like alcohol in a way. Not everyone reacts the same way, yet you can't know how you react to it, until you've been using it semi-regularly.
With all mind altering substances, you have to ask: Why am I using this mind altering substance? Why do I like my mind being altering? I think some people genuinely enjoy the stuff, and I think some people use it to escape the miserable life they are in.