A funny thing... when you are totally focused on another they become focused on themselves, too!
So why would someone do this?
Often the motives of the co-dependent are all about gaining self-pity or gaining self-respect enough so that they can feel safe and comfortable enough to embrace their own inner soul. Just below the surface is a lost and rejected child that doesn't feel that who they are themselves is worthy of love.
Some people lack in self-perception and can only identify who they are through that of a second person. They manifest 'who they are' only through another's eyes, thoughts, or views of them...and without another they are unable to find their own identity. They tend to latch onto partners because of this lack of being able to self-identify through themselves.