I'm sure legions of potheads will jump in here to tell me how pot is harmless.
I can assert that isn't true from personal experience.
In my fraternity days, as a freshman I met and ended up being friends with a guy fairly similar to myself. Bright, introverted, we got along really well for at least 3-4 months. Then he discovered the joy of weed. From the first joint in his life, he quickly progressed to constant indulgence, and his room became a haze of bluish smoke.
Finally we went on a freshman bus trip and he brought along what I thought was a huge bag - roughly the size of a softball, maybe a little larger - and he smoked that gone in about 3-4 days.
He became a different person. Distracted, apathetic, unresponsive, glassy eyed. I can't say he became stupid, he 'discovered' apparently that if you study baked and then take the tests baked, you do just fine. I think he graduated with a 4.0 anyway.
But he was a wholly different, dislikeable person. I missed my friend.
Who knows, right? Maybe he was an addictive personality that would have similarly destroyed himself in alcohol or other drugs.
All I can tell is that the person I met and grew to like was not eventually the person he became in a very short while. And that was ENTIRELY due to weed. I watched it happen.
But go ahead, keep telling yourself that marijuana's harmless, it hasn't changed you at all.