I think people are just really unprepared for the reality of mental illness. People imagine someone who's feeling a bit sad but after a nice cup of tea and a rousing pep talk from them (probably involving the line 'we all feel down sometimes!') they'll bravely wipe the tears from their eyes and say how much better they feel.
^^THIS^^
I'm sure people mean it when they say they'll be "there for you" or to "talk about it any time", but they don't actually understand what that means. It's not about "talking" for everyone, sometimes it's just about understand why a person might be a little distant, or not wanting to socialize, or are extremely emotional/lashing out and just continuing to check in on them and not get angry at the behavior.
I'm quite open with people about my battle with chronic depression, when I realize that I'm having an episode (I don't always consciously realize it, or it might take awhile) I tell my family & close friends "Look, I'm depressed right now, I don't know why, I'm on my meds, it will pass, just let me have my space and don't get upset if I'm distant or a little emotionally unbalanced right now."
DH gets it, and gives me the space I need. Some of my close friends get it, and they are fantastic, others haven't and it's ruined friendships because they take it personally, or think that I'm being dishonest about it, or don't understand why I can't just "fix it".
Mental illness is still so horribly misunderstood.