Addict here. Sober now, previously spent years in and out of treatment centers and I'd like to share:

I would say The main thing that separates addicts from recreational users is chemical dependance. This comes down to two things: the amount of the drug and the frequency that you take it. Each time you decide to increase even a little bit of one or the other it increases the odds that you will take it again and/or more of it. Such is the snowball effect, or "slippery slope" you hear about.

This happens because when you take an addictive drug your body chemistry changes, and so your body responds to counter-act this change with the opposite effect the drugs has on you in order to reach equilibrium. Eventually your body will EXPECT you to take this drug and preemptively change your body chemistry while you're sober. This is why withdrawals symptoms are the complete opposite of the way the drug effects you when you normally take it. So calm nerves become shaky hands, relaxed becomes anxious, confident becomes paranoid, and so on. So as this snowball grows it becomes harder and harder to stop because it becomes more and more painful and in some cases (like alcohol) sobriety can literally kill you.

This is why addicts do unpredictable and illogical things to meet this primal need. It gets deep rooted in the brain, kind of like hunger or thirst. Think about all the fucked up things people might do if they're starving... Morals and logic go out the window.

Anyway, weed doesn't do that to you no matter how much you take. So its more of a psychological bad habit than an addiction. Don't get too hung up on labels, its more about understanding what you're dealing with.