Exercise addiction, like other behavioral addiction is again just one manifestation of a methodology that the individual has learned that will bring about a positive effective feeling state when they engage in it.

For example, I work with what are called "obligatory runners" and these are people who need to run every day, or at least they feel that they need to run each day or they get depressed.

At some earlier stage in their life, they found that running relieved their depression and so they continue to run to mitigate their depression.

But over time it no longer just did that - now if they don't run, they will actually bring about a depression, they actually change their neurochemistry, their biochemical interaction with the world, and now must engage in this activity.