Co-Dependency is an unhealthy reliance on another person for every thought, action, and feeling. It consists of people who seem to be defined by another other person. One person relies so much on another person's opinion -- that the daily functions they once had as an individual are lost. That individual is no longer capable of making his or her own choices.
A person who is co-dependant is constantly striving to please another person and have made them selves so self-less that they begin to lose who they are. Their life becomes a sacrifice for another person.
A person who is co-dependant ceases to be them self and becomes part of two. A person's want for someone else in their life is overcome with the need to have someone else in their life in order to function. The person feels the need to spend every waking moment thinking about that other person, being with them, talking to them, or thinking of ways to make that person happier.
While a marriage should strive to do those items, there is a happy medium and for a lack of a better term, co-dependency could be considered as 'stalking'. The constantly calling them, seeing them, thinking about them, giving them things -- are traits of stalkers who feed their need off the presence of the other person.
If a person doesn't feel happy unless they are with their significant other, this is where the problem begins. To only gain a feeling of happiness by someone, an outside factor that you can't always control is unhealthy. A person must be happy with them selves and love them selves as a person, if you don't love yourself, how do you expect someone else to?