There are so many stories of that happening to people. Often it's fun when they're young and when real life starts weed suddenly isn't fun anymore.

My personal theory is that weed is basically a slight psychedelic and it's also really easy to become a bit of a non-achiever when you smoke regularly. So after years of use there is a part of you that knows you're not being all you could be and that this isn't a path to long-term happiness and then you smoke a plant that makes you more introspective and at the same time is a big factor in what is holding you back. Getting high often also suppresses feelings and after a while they come bubbling to the surface because you never deal with them in a healthy and sober state.