Note: this is to be read tongue planted, at least somewhat, in cheek.
The Golden Rule of Relationships is: Never date anybody crazier than yourself.
There are, of course, exceptions. But generally, dating someone with more issues than you isn't good for your mental health, especially if you're not not perfectly sane. Two depressives locked in a mutual downward spiral can get Very Ugly very quickly.
Of course, if I followed my own advice, I couldn't let anybody date me and maintain a clean conscience, but hey.