I've also known three people who smoked marijuana regularly and developed serious life problems as a result, along with a larger number who didn't develop major problems but who definitely "stalled".
I also knew many who smoked it regularly without major issues, so I'm not saying it inevitably causes problems, but I am 99.99% certain that if you already suffer problems with motivation or life direction, marijuana can majorly f... you up.
I used to smoke it occasionally myself and I definitely saw its positive effects - for computer scientists, it's a fascinating springboard into understanding consciousness and meta-cognition. It makes good music and art even more enjoyable.
It's also one of the most seductive drugs out there. Alcohol leads to very painful hangovers, which is a clear signal for people to slow down. Psychedelics are too mind-blowing and the trips last too long to be used regularly. Heroin, crack, meth etc are widely seen as dangerous. Cocaine is expensive.
Cannabis is affordable, and can be enjoyed on a weekday evening without strong next-morning effects. So some people end up integrating it into their routine: surfing the internet stoned, listening to music stoned, playing video games stoned, chatting to friends stoned.
If you have your s... more or less together, you can probably use weed semi-regularly without too much damage. But if there are one or more real problems in your life, things which would take real effort and motivation to fix (poor social life / grades slipping / lack career direction / etc), weed will tempt you to chill out and ignore said problems. You concoct wild rationalisations about why you don't really want X, why people who chase X are manipulated by the system, etc, etc.
Long-term, you run the risk of becoming paranoid and demotivated, struggling to make everyday decisions or to get a grip on your own thinking. Given that this isn't a universal consequence of weed usage, it might not be solely due to the weed but also certain personality traits which weed amplifies.
I rarely smoke it at all any more - personally, I found training myself to enjoy good art, and reading deep philosophy/literature gave me access to much more profound pleasures and insights than weed ever did.