My theory is that it is similar to caffeine. People not feeling normal until they have their morning cup. What is happening is they are going through a mini withdrawal and feeling the symptoms from it, and once they get their fix, they're back to no withdraw symptoms. I feel weed is similar for people who smoke daily. Your mind's baseline is that with THC and when you go without for a day or longer, you're going through a mini withdraw and need the fix to rid of the symptoms and feel "normal" again. I don't care what they say, you can certainly develop physical and mental dependency on it.

Source: I'm saying that as a current daily smoker who will quit (again) soon. Hopefully I won't get night sweats and disturbed sleep quitting this time around.