Defense mode keeps us safe. Sometimes we block out so much that we don't realize that what we're blocking has already gotten in. Anger, love, jealous, lust, etc. can ease through the tiniest crack of a well constructed wall.
For a moment, notice your reactions to certain situations. Our reactions or lack thereof can possibly indicate more about ourselves than we'd care to know. Emotions both good and bad can sometimes leave us asking, "Where did that come from"?