Open Question: Why do some people go back to bad habits or addictions when they get under stress?

How can this problem be stopped? A guy I know returned to a bad habit this week and today when a stressful problem came up. I had a guy I worked with who started using Cocaine again during stressful times in his marriage. I think he ended up in rehab about 3 times. He lost his job the last time I think.
Some guys go back to viewing porn after they gave it up. Some women go on a shopping spree when under stress.