This is because Ritalin and other stimulant ADD drugs (Concerta, Adderall, Vyvanse, Dexadrine, etc) work on the dopamine system which is not only responsible for attention but pleasure, too. The drug has the same
mechanism of action as cocaine and the result
of long-term use is the same. The drug works for a while and then the person develops a tolerance to the drug and more is needed to achieve the same result. Then the person cannot even feel pleasure without the drug.