That is the point!
Mentally healthy people are rational and reasonable (within the bounds of human fallibility!).
They are not driven or paralysed by amplified anxieties, phobias, mood swings, suicidal thoughts, depression, "black clouds", ...
Yet, often people struggle to understand and/or be empathetic with someone suffering from mental illness because they are not thinking and/or acting in a "rational" manner.
This dismissal or diminishing of what is going on can even be done by a sufferer themself, "I know I am having all these crazy thoughts and feelings, but I know they are crazy so it does not matter.... I don't need to get help."
The person is so far gone that they actually think that their irrational thoughts are rational. It is everyone else who is crazy...
Here two good recent articles about mental health by physicists.
The plight of the postdocs: Academia and mental health
There’s an awful cost to getting a PhD that no one talks about