Pot isn't physically addictive, but it can be mentally addictive. You never forget what it was like to be high, and presuming you enjoyed getting high, not doing it anymore sucks.
Here's an example: Remember that episode of futurama where Fry discovers he has a fortune in his bank account, and he uses it to buy a can of anchovies because they're rare because Zoidberg's species ate them all? And at the end Zoidberg devours the anchovies and wants more and Fry says "There aren't any more. And there never will be." Well, that's what it's like to quit smoking pot.
What a weird metaphor
