If you're suffering from depression or ADHD (or other mental illnesses) where you have low amounts of dopamine and noradrenaline (and/or other neurotransmitters, and yes, i know that it's WAY MORE complicated than just having low levels of certain neurotransmitters and that there are very likely other, different disfunctions in the brain), why taking drugs or psychiatric medications still cause tolerance, if taking them (and even illicit drugs in certain dosages) is supposed to bring those levels of neurotransmitters into the normal, healthy levels like they're supposed to?

Tolerance occurs when there is too much of neurotransmitters for an extended period of time in order for the brain to protect itself from neurotoxicity and maintain homeostasis. By bringing abnormally low levels of those neurotransmitters to the levels they're supposed to be in, the brain shouldn't be "fighting" against it and thus the tolerance shouldn't build up.

EDIT: does it mean that the brain has abnormally strong coping mechanisms and that's why the mental illness (besides mental disorders that you're born with, which are actually caused by wrong brain development) occurs? That would imply that in some cases, the brain doesn't necessarily have a problem with synthesizing and firing the neurotransmitters but rather that the neurotransmitters are degraded way too early/fast or that there's an imbalance in presynaptic and/or postsynaptic receptors. But that brings a question why there are so many cases of treatment resistancy, because MAOIs should be fixing this issue (when it comes to degradation) and overall the antidepressants are thought to work by increasing BDNF, promoting neuronal repair and neurogenesis.