There are certainly times when defending yourself and standing up for yourself is necessary. Being assertive allows you to communicate your needs to others and can set standards for the way you allow yourself to be treated. However, there are certainly times when being defensive can block a healthy dialogue.
When referring to communication, according to the dictionary defensive means, "very anxious to challenge or avoid criticism. Self-justifying, oversensitive, prickly, paranoid, neurotic and uptight."
Being defensive can cause what started out as asking for clarification to become a big brawl. It can leave you feeling constantly on the edge and even without your awareness, in constant attack mode. It also prevents you from learning and growing. If you notice yourself often in defensive mode, it may be good to check in by finding out what is going on with you.
Here are some signs you may be in defense mode:
Slow to listen, quick to speak: not giving others time to express their concerns and perceptions, but instead talking over them and being quick to prove your point. Or, listening to respond rather than to understand.
Assuming the worst: constantly thinking others are against you and waiting for you to mess up/fail.
One up attitude: constantly feeling like you should have the last say/word, or be one step ahead to "check" others.
Lack of tolerance: not being tolerant of differences-- feeling like your way/beliefs are the only ones that matter and therefore feeling threatened if differences exist.
Unhappiness: there is the saying "familiarity breeds contempt," but is it familiarity that does this, or unhappiness? I would say unhappiness. In a state of unhappiness, it is not unusual to feel as if life, the higher powers or whoever is against you and is messing up things in your life. Therefore, causing you to be on guard and inciting defensiveness-- projecting your unhappiness on to others.
Check in with yourself by finding out what is the bigger or deeper issue, if you find yourself constantly in defense mode. Address the root issue so that you can dialogue with ease and therefore improve the quality of your relationships.