The best definition I've heard for "addiction" is this: Addiction is when a specific behavior persists despite mounting negative consequences.
Note that there is nothing in that definition to distinguish between physical and psychological withdrawal events (if any), and also note that negative consequences are a key part of the definition. If you are using something repeatedly without negative consequences, you're not addicted. Finally, it's quite possible to get addicted to anything, and yes, genetics (and possibly environmental factors) can be a huge determining factor.
Now, with that being said, there are often severe negative consequences which you are simply not aware of at the time. As mentioned, the contents of pot smoke will fuck up your lungs nicely, and the last time I checked, lung cancer was a pretty big negative consequence. I've also heard about studies which indicate that long-term pot use is tied with a serious amotivational syndrome -- you just don't feel like doing a damn thing -- leading to depression. Last but not least, I know people who can't get certain jobs they are otherwise qualified for because of a history -- even one or two times! -- of pot use. I'm not in a position to tell anyone how they ought to weight their short-term satisfaction versus their long-term job options, reputation, or health, but I suspect that people in general don't think about the consequences of their behavior a few decades from now.
Just to blow up one particularly stupid argument: Saying that pot is less addictive than cigarettes is NOT a meaningful statement. According to what I've read, nicotine addiction is second only to heroin addiction in difficulty to break. I would never wish either on anyone.
America needs to figure out what the balance is between individual rights and the good of society for once and for all. Every argument you can use for or against pot can be used almost verbatim if you substitute "alcohol" for "pot," with the one obvious except of legality. However, as noted by some other people, the issue is that the same kind of reasoning ("I'm an adult, this is my body and my choice") can be used for almost anything else which is potentially bad: cocaine, ecstasy, meth...hell, how about fast food? It's YOUR body and YOUR choice about what substances you put into it...until you smash into MY car on the highway, or MY face gets smashed because you're hopped up on speed and paranoid, or MY insurance rates go way up because YOU are in the hospital with a heart attack. It's a much larger issue to address, and hippie arguments about "but it's NATURAL!" are just (pardon the pun) a smokescreen around the core issue of rights vs. responsibility.