A mirror shows a crude reflection of our physical form. That image, being static, will not reveal deeper aspects of our selves and our connection with Krishna, or relations with others. Only in the reflective dynamics of relationships with others are we truly confronted with our conditioned self in order to uncover our true selves.
Dealings with others are great opportunities to become aware of our blind spots. Often those close to us serve as a perfect mirror pointing our issues. Our parents, spouses and children will continue to press every button we have, until we realize what it is that we do not want to know about ourselves yet. Krishna works through them, showing all the dark places of our conditioned nature in order to enlighten, purify and set us free.
Another type of mirror is the eyes of someone mature and wise who fully accepts, love and appreciates us. Such a person can usually perceive our talents and virtues. Without that mirror, our image of who we are and what kind of gifts, and abilities Krishna placed in us can be unclear and distorted.