Why do some people who have heart transplants also develop personality changes?

I suppose the first question is "Do they?"

After coming near to death and been given some sort of reprieve they might have a somewhat different view on life. They may well have a different view of what is important in life.

I haven't had a heart transplant, but have been ill with chronic heart disease that required a relatively minor heart operation (fitting a stent to an artery) and enforced early retirement.

I believe that my underlying personality is unchanged, but my attitude toward family and friends is more relaxed and mellow. Also my work which occupied almost all of my waking moments in thought and deed now seems so less important. People seeing these changes might describe them as a personality change, but I see it as an adjustment in attitude.