Emotional dependency involves resting your sole happiness on the energy of another. It can be extremely draining and can so easily occur in relationships of any calibre.
Leaning on another during any relationship is expected as it provides the opportunity to be given unconditional support. The possibility of resting to much on another however, is what can lead to the relationship falling apart. Even worse than that, is that a person can be left to feel worthless and lost as they do not know who are they independently of the other person.
How to Keep This From Happening To You
Whilst there is a 'Honeymoon Period' in any relationship worth having, it is obligatory not to forget who you are as an individual. So many times has a person taken the easy way out and gone into a relationship to overcompensate for the lack of individuality (and so soon do they find themselves single again, more confused than ever).
If you do not know who you are outside of a relationship then take some time for yourself to figure it out. Concentrate on building a stronger person within yourself before you transgress all that confusion onto your partner. Without finding out who you are first, a relationship is doomed to a state where happiness rests on the state of another person. Relationships are healthy and make us grow yet only when two separate entities can be as much true together as they can apart.