I think this is sometimes referred to as the "identified patient" situation. Because one person has a diagnosable condition, they are automatically the "sick" one, the one who has "issues". Of course it doesn't happen in every relationship or with every person who has a mental disorder, but it does in fact happen and can have a negative impact on relationships. Sadly, it can prevent the "well" person from admitting to their own faults as its far to easy to say "you're the one who is depressed/bipolar/whatever" as there is "proof" of being "abnormal" because of a medical diagnosis. It really is sad, and as a single person who has a disorder that I will deal with for life (in varying degrees), I am careful to only keep people around who are compassionate and understanding.