Although the physical dependence of the drug may not be as addictive as other hard drugs, marijuana can be addictive. Like any progressive drug use, one may be psychologically dependent on marijuana as more usage is consumed. Continued marijuana use has been documented to alter brain function and rewire brain function.
While Marijuana can become a habit, ingrained into your daily routine, this does not make it an "addiction" in the sense that we typically use the word. There are no physical withdrawals to the drug, which are common in other types of addictions. A heavy user may feel a little uncomfortable if they don't smoke, but that's because they have become so accustomed to living a certain way, and is not indicative of a physical dependancy. This is similar to how I would feel if I poured my morning coffee, and realized we were out of milk. Stinks, but in the grand scheme of things, it's not a big deal. Your average marijuana smoker will never feel any amount of withdrawals or any other symptoms associated with addiction. Any perceived addictive tendencies can be attributed to the amount of use. Anything we do in excess becomes habit, and habits are hard to break. But, that does not necessarily qualify said habit as an addiction.
Marijuana is not addictive by any means. The only thing that may be addictive is the way you feel, but that is no different then people who like to run and get "runners high". There have been many scientific studies on marijuana and no addictive properties have ever been found.