To be a good/helpful/nice/whatever part of a couple you need to be ok yourself. If you don’t take care of yourself you can become tired, crabby, sad, depressed or anything else and all of these things are not good for you, and not good for your relationship. I know its not rocket science but honestly I promise you water, exercise, and eating right (or even better) really do help.
If we value those we are in a relationship with we need to value ourselves.
Check out this post on “Stress and Life” (by my Mom) for a little more insite on how it impacts not only our relationships but our physical selves in not so great ways.