It's a strange thing to say.
"What do you mean, 'cute'? No one thinks mental illness is cute!"
And I don't think anyone puts those words together. But people sometimes act like it.
Mental illness is not a quirk. It is not a spice of personality, its not a "thing you do" and it sure as shit isn't as cute a manic pixie girl would lead you to believe. It is not "breaking from the mold" and it is not "being a free spirit."
The term "mental illness" itself is so broad as to be nearly meaningless. The severity ranges from tolerable to completely life wrecking. Some can be managed with pills, some with therapy, and some can be handled personally, all of which varied greatly based on the kind of illness, the person, their support group, their life, what causes it (such as PTSD), and just about everything in existence. What works for one person doesn't necessarily work for another.
We tend to think of depression and anxiety when we talk about mental disorders. I hear much less about the more severe forms, like psychosis, schizophrenia, bipolar, manic depressive, and OCD--not the "I like things in order" OCD, but the serious, "I have to turn the lock 20 times and click my tongue" sort of OCD.
Not to say depression and anxiety aren't terrible enough, but the less talked about forms of mental illness can seriously wreck someone's life. Imagine hearing multiple voices that tell you people are out to get out. That the pizza guy poisoned the pizza. That the phone ringing is the government out to get you. Imagine literally losing your mind and coming back into normal consciousness after you've been arrested and put on meds again.
Don't think that your love can cure them. Step away from that Hollywood bullshit. Your friendship will be appreciated. Your love will be helpful. You're love can be a light in their darkness.
But you will not cure them with your love.
They will need more love and patience. They will need more room for understanding and acceptance. Your companionship can help them grow and can help them cope.
They may do things you don't understand. They may experience emotions in a way that is counterintuitive. You may not understand why they push you away but think nobody loves them.
YOU WILL NOT CURE THEM WITH YOUR LOVE.
I'm not saying to put up with their bullshit if you don't want to, or if they are abusive or manipulative. I fully support when someone understands that they can't handle the person or the situation and needs to walk away.
What I'm saying is don't walk into it thinking their illness is cute. Don't think you're one snuggle fest away from curing them. Don't tell yourself that you know better and with a little elbow grease you'll fix them right up. Don't think that you're Mary fucking Poppins and a spoonful of sugar will do the trick.
Mental illness isn't cute. You can't cure it with your love.