The first time I was hospitalized in 2007 a friend had heard about it. But he didn't know the reason I was in the hospital. When I told him, he said "At least it's not cancer." I didn't say anything to him. But later I would become very upset about his comment. Mental illness is just as serious as physical illnesses. You can die from mental illness just like you can die from a physical ailment. I never did say anything to my friend. Fast forward six years. When I was hospitalized for the second time in 2013 I finally said something. I even told him about the high rates of suicide and mania-caused lack of impulse control. He apologized. And of course he didn't remember saying this to me. I'm glad that I finally said something.
Around 2007/2008, I went to visit a few friends from college who were in law school. One of my friends was telling us about one of her classmates who had a nervous breakdown. She ended the story by saying, "Well, not everyone can cut it." I was highly offended by her statement. But this friend did not know about my own struggles with bipolar disorder. And again, I did not speak up. I regret this to this day. I have never called her out on this.
When I was depressed in 2006, my mom snapped her fingers in my face and said "Snap out of it." If only depression worked that way. I knew I was being irrational. I knew I was harboring negative thoughts. But I was powerless to do anything about it. Especially since I didn't receive treatment (therapy or medicine) for the depression.
I give you these three examples to caution you against making insensitive comments. You never know what another person is going through. I do recognize that sometimes the comments are very well meaning. And let's be honest, it is hard to know what to say all the time. Sometimes you don't need to say anything. Your presence can be enough.
Try to treat people kindly. "Everyone is fighting a battle you know nothing about."