There is a difference between being apathetic and insensitive or otherwise desensitized. If you were to turn on the news and not come to the realization that a possible imminent war is a problem, that would be an example of apathy, a complete and total lack of emotion. If you turn on the news and realize it is a problem, yet are unwilling to do anything about it because of inconvenience or the trodden thought of not being able to do anything, that would be an example of desensitization. You are still feeling emotion, yet do not care enough to do anything about it.
Apathy:
One of the main causes of apathy in today's world is PTSD or Posttraumatic stress disorder and severe depression. The obvious cause of PTSD is experiencing a horrific event or sequence of events in ones life. The event can and in most cases does affect the mind so that it will slow the production of certain chemicals related to what we call "emotions" such as serotonin. A few major medical drugs prescribed to help sufferers of PTSD and depression, such as Fluoxetine (Prozac, Sarafem) are SSRIs (selective serotonin reuptake inhibitors) which compound the effect of serotonin loss though in a different way.
The effects of apathy are noticeable only if you knew the person beforehand. People that suffer from being apathetic usually tend to be very balanced, logical people and as I have personally noted, overly analytical of every day situations.
Fact: A U.S. Military study reported that 1 in 8 soldiers who fight in Iraq will return home with or become afflicted by PTSD while/after serving duty.
http://www.msnbc.msn.com/id/5334479/ns/health-mental_health/
Desensitization:
I believe the main cause of desensitization, especially to war and various problems in the world is general media. I'm assuming most people reading this watched the news with their parents when they were little or were allowed to watch cartoons. I remember the innocent, pure cartoons of my childhood quite fondly, where Wiley Coyote continuously tried to kill the Road Runner using TNT, rockets and just about every trick in the book. Or where Elmer Fud hunting shotgun in hand relentlessly tried to murder a talking rabbit. And don't even get me started on the news! Violence, especially violence, is prominent all over the world and has been since the beginning of recorded history. I believe desensitization is a natural coping mechanism that allows us to function efficiently as both human beings and a species. After all, if we weren't desensitized, if every act of violence had a major effect on all of us, the world would be in a constant state of chaos because most humans are quick to anger and slow to forget.