Someone exhibiting co-dependence tends to forget about themselves completely and begins to think that they have to take care of others to the exclusion of our own needs and wants. There are very good reasons to care and help others at times. No one owes any one anything. We choose when it makes sense to help and when we want to do it. This is what it means to have good boundaries.
Boundaries are where one person ends off and another person begins.
It is the knowledge that we are responsible for ourselves and that we are responsible for taking care of ourselves. Often people allow themselves to be abused because when they don't help another person that person tells the other person they are selfish to get his or her own way. This is abusive and the person who used the word is demonstrating the true selfishness. When most people hear the word abusive they tend to think of legal abuse. This means that when people are rude, try to hurt, or are neglectful of how another person feels they are acting in an abusive fashion.