I dont care what you have to say, tons of people get withdrawals. There is threads on this very site. You obviously have never heard of Mental addiction. its not physical. Its the fact that i smoke so much weed when my bodys without it, it wants it. My body is used to being high. I also have "Stoner stomach" not a actual "disease" or anything like that but when someone(not everyone) smokes lots of weed before meals, constantly it throws off your stomachs "clock" and you don't feel hungry unless your high. and if you smoke before bed all the time also like me it throws off your sleeping "clock" and make you sleep for shorter periods of time. As i do.
Like i said this doesn't happen to everyone but a good amount of people. You need to start learning about the human body and mind since you want to be a chemist, you need to learn about the chemical reactions that happen within your own body and mind.