1.First of all it lowers your intelligence level permanently, while your brain is still developing, you suffer a significant reduction in intelligence that follows throughout life.This may explain why people who use this drug often seem to fail to develop successful careers.
2.It denies you a serious job,most of them are drug tested and upcoming exams so you can't concentrate long enough,it affects your communication in such a way that people will think you're uneducated and not someone they can rely on.
3. It costs, it's expensive, depending on how much and how good of weed you smoke it could cost you about $15-$100+ a month.So instead of spending that money on weed you could buy something else or you could go to gym.
4.It is a stress relivier but makes you less ambitious,makes you a lazy person,it's really easy to lose track of time when you're high, keeps you from living fully and leads to harder drugs, which MIGHT lead to mental illness
5.Weed is illegal,to get it you're going to have to break the law and become part of black market.This will put you in contact with dangerous people.Since it is illegal and it comes with some steep penalties they will have no problem busting you.