I think there are a few facets to it, so i don't claim to have all the answers. But one I see is that with the parenting/society we have now. The always praising children and giving "participation trophies" leads to very little disapointment emotionally in a child/ teenager's life. They have not learned to address those emotions when they are 5 and are harmless. They are experiencing those emotions for the first time and have not bult up the required coping mechanisms to not self destruct. When you hear of one of these, just think of it as a 3 year old being told NO he can't have a candy bar.
Some have mental defects that never allow them to cope but most of it is a learned behavior.
I was always the bad guy in raising my daughter. My wife and I were just talking about it the other day. She asked me how I do it. My answer was simple " i put my feelings of guilt aside, make the decision based on long term goals not instant gratification (mine and my daughter's)".
As a father of a young female in college my biggest and i do mean my biggest fear is she date someone who can't let go if the time comes.