If they're caused in the brain, they're deemed by natural selection to be necessary for survival. Anger is drug related. It also causes problems, and shortens life. And I admit, there was perhaps a slight mis-wording in my first sentence, quickly rectified in later lines, however.
So where do you draw the line between 'necessary for survival' and not? Opiate painkillers are helpful for making people not go into shock and also for being able to act as though they're not in crippling pain, but most certainly reduce cognitive capacity.
And then there's anti-depression drugs where the effect is somewhere even less measurable, and don't at all directly increase survivability. Indeed, the only thing they really affect is state of mind to be more beneficial.
Plus there's chemicals in various food products that could be done without but bring on pleasant effects but have negative effects physcial or mental such as high sugar and fat foods, caffeine, and capsaicin.
And yes, all of these things are as addictive as alcohol or marijuana (actually caffeine more so since it's chemically addictive as well) because people can be psychologically addicted to anything enjoyable.
So yeah. That definition of drugs excludes a lot of things.
EDIT: Hidden twist, Leo's list actually contains every GD regular and it's only short relative to the entire Bay12 Forums member list.