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Those who base success on their own work and believe they have control of their own life, operate in an internal locus of control.  On the other hand, those who consider their success or failure to outside influences have an external locus of control. 

For example, if you receive a promotion, and you have an internal locus of control, you probably believe that this is the result of hard work you put in.  On the other hand, if you have an external locus of control, you might attribute it more to luck, fate, timing, or some other element out of your control. 

Wellness absolutely involves your own actions.  What we choose to eat, how much we exercise, and the way we interact with others are often things that are in our control.  Therefore, an internal locus of control is more analogous to the principle of wellness.