I often read about and research missing people's stories. They're always sad, heartbreaking, and painful especially for those who care about them and who are left wondering what happened.
I've noticed that "depression" in one form or another often seems to lurk behind (or in front of) the various disappearances.
Why are there so many of these sad stories...
Those who are trying to deal with their various forms of depression sometimes end up being more vulnerable to:
self-harm or suicide
being harmed by others
not making the best choices (of one kind or another)