Very hard in what way?
Mental health issues, depending (and we have a vast array of them) can cause severe to moderate to barely any issues at all when it comes to communicating, maintaining and even being able to begin a relationship. Consider that you're depressed and suicidal, burdened with the thoughts of how terrible and horrible you are. That makes it difficult to be with other people (to many depressed people, the feelings of alienation and isolation makes for a valid reason to withdraw from social interactions) and it's definitely not easy to start anything new and romantic. Then we have paranoia, delusions of various kinds, psychosis, schizophrenia... Overall, a lot of mental illnesses really dampen your lust and interest in doing anything, and particularly in being around other people.
So there's one way to look at it.