The following is some information on exercise addiction: "Exercise Addiction: When Healthy Becomes Hazardous."
I found this extremely interesting because at times I definitely struggle with compulsive exercise. For me, I tend to write a training plan and then become compelled to follow it to a T regardless of how I'm feeling. In the past, this has meant I train through injuries when I should be cutting back, skip out or cut short family and social events to get in a workout, and over-train.
The article points out the potential of addiction when it comes to exercise. Vigorous exercise over an extended period of time releases chemicals like dopamine which create a sense of well-being. It's easy to become "addicted" to that feeling and constantly work at chasing that high.
As long as you're mindful of this fact and realize that you're not going to have those euphoric feelings every time you exercise, you can avoid exercise addiction. If, however, you're constantly working to obtain that high, you may become dependent on those feelings and end up in a pattern of addictive behavior.
As the article points out, mindfulness is key. Realize when your life is being negatively affected by exercise, and work to restore balance.