Persons with mental illness can usually relate to each other on a common wavelength. To illustrate this, I will bring up the analogy of my dog and cat. They are of different species, and yet, when the dog was introduced, my wife and I taught them to get along with each other.
They seem to have nonverbal communication in a language that human beings don't understand. One might expect that there would be a language barrier between dog and cat, but this does not appear to be so. Because they are together and are essentially in the same situation, they relate on their own common level.
People within the mentally ill population vary across a wide spectrum of functioning levels and socioeconomic backgrounds. It is not unusual for someone with chronic mental illness to have a Bachelor's Degree. There are others who may not have completed high school.
In my case, I took the California Proficiency Exam that existed in the 1970's and 80's that allowed me to graduate from high school early. I then became mentally ill, but later completed trade school for electronics. After that, I continued learning by becoming self-educated with only a small number of college courses. I have educated myself in writing, in critical thinking, in meditation and in computers. (I have also taken some writing, computer, and meditation courses.)
Yet, I relate on a common level with mental health consumers who have had less education than I, and with those who have more education. Persons with mental illness tend to have a common bond--most of us having suffered the same hardships.
Romantic partnerships between two persons with mental illness are not always doomed to fail. In my case, I have been married for seventeen years, which is better than many people who do not have a psychiatric disability.
Sarcasm is a prevalent trait among mentally ill people. Gallows humor is therapeutic for people who must deal with difficult realities. "Bumming" cigarettes is common, but maybe less so than it was in the past. There is promiscuity among some persons with mental illness, but I suppose the same can be said for people at large.
People with mental illness have parallel characteristics that might spread by virtue of osmosis. For example, we may feel that we are succeeding in an employment situation, might get a boost of self-esteem from that, and might foolishly believe that we can go off medication. Other similar forms of denial are common, and they lead to the "revolving door" of being in and out of institutionalization.
I once lived in an apartment complex set aside for persons with psychiatric disabilities. When I moved out of there, and I don't recall my exact reasons for doing so, life was harder for a number of years afterward. But now, looking back on it, the struggle to survive in various situations has made me a better, stronger person. Refusal to be defined by the illness, while at the same time getting the illness dealt with through medication and therapy are two things that can help get someone out of the typical frame of mind of persons with mental illness--and this could be a good thing.