To be clear, people respond differently to different stimuli, and response to MJ is no different. MJ is certainly NOT for everyone, and can be abused. Addiction takes one of three different basic forms, (physical, physiological, and psychological), and marijuana can fall into the last group, (but so can watching too much TV, snowboarding, golf, whatever...). The point is anything someone likes too much has the propensity to become psychologically addictive. To say marijuana is addictive in the sense the drugs mentioned below are, is a myth.
Caffeine, nicotine, cocaine, and many other drugs are physiological in their addictive properties, and produce side effects of physical discomfort when someone is experiencing withdrawal from those drugs. Pot does not fall into this category. Nicotine is extremely addictive in this area, by contrast.
Alcohol, (yes, alcohol), heroin, and other opiates, are physically addictive. The human body actually builds a dependency upon these. Many older alcoholics actually die in hospitals, (for treatments non-related), because alcoholics are denied alcohol during their stay, and die of "complications", (actual alcohol withdrawal). Alcohol also contributes to sugar addiction which leads to type 2 diabetes, hypoglycemia, etc. Alcohol IS the ultimate gateway drug.
Marijuana abuse is over-hyped. Sure, there will always be people that cannot be balanced about using or doing anything they enjoy. This almost always points to something else wrong in their lives that they need to address, and as a result should avoid things they have the propensity to abuse. However, to put marijuana into the same category as physically or physiologically addictive substances is either ignorance on the part of those repeating this rhetoric, or outright misinformation campaigns from those with an agenda. It is what is known as "junk science".