system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is it about mental illnesses that makes them so hard to cure?
gpt
We don't understand mental illness as well as we understand physical illness, because the mind is a much more complex system than most individual body systems. We have some theories, but for the most part mental health care is largely based on past trial-and-error to see what works.

There's also a drug-development challenge: there is a barrier called the *blood-brain barrier* that makes it hard for most molecules to enter the brain. Many medications that would do something if we could get them into the brain are hard to get there. The space of possible drugs that you can explore for mental health is often a lot smaller as a result.