The subjective "high" caused by a drug is determined by how fast the concentration of dopamine increases¹ in the brain. Each individual drug has a distinct mechanism of action, so they all increase dopamine concentration at different rates. The high is also linked to the route of administration of the drug; the fastest routes are intravenous administration and inhalation, followed by intranasal administration.
Prescription stimulants are designed¹ to modulate neuronal circuits in a healthy manner. Recreational drugs do the opposite. They aim for a massive release of dopamine in the brain's mesolimbic reward system. All drugs that can lead to addiction have this effect.
High concentrations of dopamine results in neuroadaptation² in those circuits. Their receptors are down-regulated so as to better tolerate the drug. Less receptors for the same concentration of neurotransmitter translates to a diminished response. This is why the first use of the drug provides the strongest stimulation. It also explains why addicts may continuously escalate the dose; they're attempting to get that same level of stimulation again.
Neuroadaptation is also associated with withdrawal effects. After down-regulation occurs, the new amount of receptors is appropriate for higher concentrations of the neurotransmitter. After the drug wears off, concentrations are back to physiologic levels which are no longer adequate. The resulting neurotransmission deficiency results in withdrawal effects. The flow of information within the affected circuits has become dependent on the higher neurotransmitter levels³ caused by the drug.
>How bad do the consequences have to be before a behavior becomes pathological?
In the development of addiction, a dramatic change in behavior can be observed: addicts gradually shift from impulsive use of the drug for pleasure and reward to compulsive use in order to avoid the adverse effects associated with withdrawal.
These impulsive-compulsive disorders are proposed¹ as the common neurobiological substrate of many disorders, not just addiction. They're also linked to OCD, gambling, obesity and others.
¹ Stahl's Essential Psychopharmacology. Chapter 12 and 14. http://imgur.com/a/LcgkT