Well simple, your body gets familiar with things if someone is taking it for the TCH “high” then slowly as time goes you develop less of a effect over time. The only way to get the same “high” is too take more this leads to harder drug use. Another interesting concept is that during the drug use dopamine is released, but after your first “high” your body will anticipate it and will release dopamine before use. Kinda like how when a child is going to be tickled they giggle before you even tickle them.