The difference between physical and psychological addiction is that things that cause physical addiction are actually addictive themselves — that is, using them tends to create dependency as a matter of course — whereas psychological addiction depends on the user's state of mind, and people without that state of mind can use them safely. You can be psychologically addicted to virtually anything.

When people point out that marijuana isn't physically addictive, the point isn't that people's psychological addictions aren't serious. The point is that they don't appear to reflect a danger posed by marijuana.