Lack of trust produces a caution that undermines the immediacy that is important in our attachment to others. As we find in George Eliot's Middle-march: 'He distrusted her affection; and what loneliness is more lonely than distrust?' Mistrust isolates you completely.
When you demonstrate trust in someone, you become vulnerable, and when you demonstrate trust regarding something or someone important to you, you become extremely vulnerable. If you confide in them, you lose control over that information.