(pre-comment: if you've never personally dealt with depression, you may not understand)
One of the things I'm coming to find in this life is that, at the end of the day, we're all alone in this world. Even if we're surrounded by friends, they're external to our lives and will never be inside the darkest of places: our own heads.
We see this play out in many ways. Maybe it's the unintentional neglect of a friendship that highlights loneliness. Maybe it's the negativity of "friends" in reaction to new ideas. Maybe it's just that inner voice, reminding you of the darkness within. No matter how you cut it, we are all alone with ourselves.