Not taking mental illnesses seriously subjects the millions who suffer them to unfair stigmatization and neglect.
You are not going to fix someone's mind by suggesting they exercise. Nobody is going to drag themselves out of bed because you gave the classic advice to "be more social" or "take a walk." As if a stroll around the lake will erase suicidal tendencies.
For as well-intended as these suggestions are, they are ultimately harmful and show a complete lack of understanding. They may be solid overall advice for life, but those with neurological disorders are not simply having an off day, or week, or month, or year, or lifetime. There are some serious chemical imbalances at work in the brain.
You may not have to wear a mask near them, but people with mental illnesses are just that: ill. Sick with something that is not avoidable, not ignorable and, most importantly, not their fault.
Don't joke about being "so OCD" because you have meticulously arranged your desk to the person whose hands are literally bleeding as a result of their obsessively washing them clean. You are not depressed because your favorite TV show ended. The person who is desperately trying to find the will to get out of bed, to get something to eat, to live is depressed. The two are not the same.
These examples minimize the experiences of those who live with a mental illness.
Naive attention-seekers and unsympathetic "pick yourself up by the bootstraps" morons have perpetuated a skewed reality that leaves the majority of those with a mental illness unwilling or incapable of seeking help. According to a report from the World Health Organization, over two-thirds of people with a mental disorder never seek help from a professional.
Even trying to relate with personal anecdotes is just misunderstanding and belittling:
"Oh, you're feeling bad? I felt bad once, it was terrible. I know just what you are going through."
You wouldn't try to relate to an amputee by mentioning your broken arm, would you?