Why do some veterans have a hard time readjusting to civilian life while others make the transition with little or no difficulty?

I think one thing that happens to service members is that they drink the kool-aid and become institutionalized, part of that "IMA soldier 24/7" mentality.
We are all headed back to the civilian world, unless you die in theatre or a training accident.

For one thing, SGT Jamell Culbreath, disabilities play a very important part as to whether one's transition is easy or difficult.
I, for instance, was discharged with a 100% service connected disability: mental illness/stress disorder. I have also recently been diagnosed with PTSD.
So, physical disabilities, mental/emotional disabilities: whether there are invisible injuries or visible injuries; they can greatly impact one's progress in transitioning back into civilian life.

In my case I suffered from a loss of identity and purpose. I had tied so much of my identity to my job that when I got out I didn't even know who I was anymore. I was able to pull myself out of that, but I very easily could have gone the other way.