Trust is vital to any healthy relationship albeit family, friend or other, society. Even in marriage, trust trumps marriage in terms of faith and commitment. From George MacDonald:
To be trusted is a greater compliment than love.If trust exist and is then broken or violated--as can and does happen--getting it back is hard, maybe impossible, but part of trust is understanding that it is never the be all end all; it is not self-sustaining or sure to hold at all times without some deeper commitment, perhaps love. With commitment, one is able to trust and keep trusting in something, if not that "someone", at least trust, a commitment to something that gives help and hope, lending truth amid much false, fraudulent, and flawed information and belief.
Why do we trust? Because otherwise there is only fear.