In order to be able to get close to other people (and to have a functioning relationship), sometimes conflict is needed. You and your partner need to be able to talk things through, even when they are bad and painful. If you or your partner gets on the defense on every argument, that means that you can't solve the problems that are plaguing your relationship.
If you want to solve this problem, you need to make a promise to yourself and to your partner (or vice versa if they are the ones that are always going on the defensive), that every time a problem happens, you are going to give your partner the benefit of the doubt, and try to talk things through before going on the defensive. That is the only way to solve this problem once and for all.