What makes addiction dangerous is that it is, in Anders Sandberg's felicitous turn of phrase, a "motivation hazard".
We have feelings for a reason. We do not work, as organisms, without our feelings. That which we do not do out of unthinking habit, we do, 99.9% exclusively, because of feelings. Feelings are part of the feedback loop of human behavior. That feedback loop works as follows: we encounter stimuli in the world; we have feelings, big and little, in reaction to those stimuli; we pick/emit behaviors in reaction to those feelings.
So it comes down to this. Imagine your left hand on a hot stove. Normally, you'd feel pain, and you'd snatch your hand away. Now, imagine in your right hand, you have a magic box with a button under your thumb. So long as you have that button pressed, not only can't you feel pain in your left hand, you actually feel pleasure. It feels really nice. Now, not only can you not tell that you're doing tremendous damage to your left hand, you have a positive motivation to keep doing it. In fact, if the button was pressed before your hand came to rest on the stove, and you didn't notice where you put your hand, you might never know how much damage you were doing to yourself. Indeed, if that happened, and you momentarily lost a grip on the button, you might find yourself in such pain that your first response would be to -- press the button again.
Our emotional discomfort is what tells us that our environment is in some way harmful[*] to us. If you use a chemical intervention[**] in an otherwise healthy person to disable emotional discomfort, they will stop taking positive actions to get themselves out of environments that bode harm. They will stop taking their hand off the burner.
And when that intervention is delivered in response to discomfort, it is conditioning[***]. The dumb yet amazingly in-charge part of your brain starts associating the feeling of emotional discomfort with the relief provided by the substance. Operant conditioning. Instead of thinking, "I don't like how he talked to me, I should confront him on that", one thinks, "I don't like how he talked to me, I'll have a drink". Instead of thinking, "My apartment sucks, I should move", one thinks, "My apartment sucks, pass the crack."
Substance abuse short-circuits the feedback loop of motivation. This is why substance dependence, untreated, is a progressive fatal illness. I'm not just talking about the direct effects of the substance of abuse, e.g. cirrhosis of the liver in alcoholism. Allowed to progress unfettered, not only will the patient lose motivation for doing things to make their life better, they will lose motivation for doing things to make their life continue. Like eat. "I could spend this $5 I just panhandled on food, or I could spend it on booze...."
So this is what the problem can be. Not how much or when or which substance or even if it is a substance, but rather, instead of what and what's the result.
[* To a first approximation, what we have physical pain for is to report physical damage happening, and what we have emotional pain for is to report the threat or increased likelihood of physical damage happening. Of course, we sometimes malfunction, and the modern world can be confusing to our ancient bodies. Also, sadness/grief may be an exception.]
[** Or, you know, sticking an ice pick into their pre-frontal cortex by way of the orbit of the eye socket. One of the classic negative outcomes of prefrontal lobotomies was pathological apathy. We know much of what we know about the role of emotion in regulating behavior due to the study of people with brain traumas, whether injury or treatment.]
[*** My psychopharm prof was a substance abuse specialist, and he pointed out that the difference between a drug with a high abuse potential and one with out was how long it took to work. An SSRI can take up to seven weeks to actually change your mood: there's no way the dumb part of your brain will figure out that your antidepressant makes you feel better, and so won't be conditioned to associate it with relief. This is why nobody is knocking over pharamcies for Prozac. On the downside, that's why it's so freaking common and easy for people with depression to stop taking their SSRI. Contrast to immediate-acting anxiolytics, e.g. Klonopin, which are massively conditioning, and thus have a vast illegal market. Patients prescribed Klonopin do not forget to take it, and they generally never miss their psychiatrist appointments, without attending which they cannot get the next month's prescription.]