Drugs = Misery.

a slight elaboration on what kasey said with less specific terms, from another psychology student. weed is not physically addicting, the same as other drugs, but it has addictive tendencies. thc when it gets to the brain, emulates the chemical released to make you feel good/happy/relaxed etc. which gives you the high state. problem being, that the body is constantly seeking homeostasis. after a while, your body realizes that you are giving it something that makes it happy, relaxed, etc, without its help. your brain decides that since its getting these chems anyway that it doesnt need to make them itself anymore, so it reallocates the energy it would have spent to create these chems to something else. so when you try to stop doing the weed, your body is more inclined to want that drug again so it still wont have to make the chems. and it takes a little while for your brain to decide to allocate that energy back to the process.

tl:dr, it may not be addicting the way most things are, but that does not mean, its not completely incapable of causing addiction.