I find the expectation that you must be perfectly stable prior to entering a relationship unrealistic and shortsighted. Of course mental illnesses can be difficult for a partner to deal with, but that does not mean a depressed or suicidal individual must resign themselves to being alone.
I don't often hear people say that those with other health difficulties should avoid relationships altogether indefinitely. That's not to say it doesn't happen, I have certainly seen this belief towards disabled and chronically ill people too. However, I have noticed that this rhetoric is most frequently applied to mental illnesses. The expectation is that we must "overcome" our mental illnesses first, which neglects the fact that they are illnesses. Not only that, but mental illnesses can be chronic and treatment-resistant just like other types of conditions.
What about those who are healthy when they start a relationship and acquire illnesses later down the line? Should they just up and leave their partners? Anyone can become ill, no-one is immune to all ailments. I wonder if commenters who are physically and psychologically healthy would practice what they preach if they developed a debilitating condition or suffered a dire change of circumstances leading to suicidality themselves.
Should a person with heart disease "overcome" their heart problems before they enter a relationship? How about a person with asthma or diabetes? No? Then why is it acceptable to make such demands of those with depression, bipolar, schizophrenia, complex PTSD etc? It's a massive double standard.